Add plugin host architecture and Storm Sewer add-on package

Generic plugin runtime (src/plugin/, HostSession, per-tab plugin_state, command router). QGIS-style add-on layout with plugin.toml and single PluginRegistration. Storm Sewer refactored as reference consumer with dispatch.rs, catchments, LandXML, headless tests (43 passing). Core update.rs decoupled from storm_sewer via CadCommand object-pick hooks. Docs: plugin-architecture.md, plugin-template, PR and issue-78 reply drafts.

src/app/cmd_result.rs

@@ -251,6 +251,11 @@ impl OpenCADStudio {
             }
             CmdResult::ReplaceMany(replacements, additions) => {
                 let label = self.history_label_from_active_cmd(i, "FILLET");
+                let was_catchment = self
+                    .tabs[i]
+                    .active_cmd
+                    .as_ref()
+                    .is_some_and(|c| c.name() == "SS_CATCHMENT");
                 self.push_undo_snapshot(i, label);
                 for (handle, entities) in replacements {
                     self.tabs[i].scene.erase_entities(&[handle]);
@@ -265,6 +270,10 @@ impl OpenCADStudio {
                 self.tabs[i].scene.clear_preview_wire();
                 self.tabs[i].active_cmd = None;
                 self.tabs[i].snap_result = None;
+                if was_catchment {
+                    self.command_line
+                        .push_info("Catchment tagged successfully.");
+                }
                 self.refresh_properties();
             }
             CmdResult::ReplaceEntity(handle, new_entities) => {

src/app/commands.rs

@@ -28,66 +28,11 @@ impl OpenCADStudio {
             return Task::done(Message::OpenPathPicked(Some((path, size))));
         }
 
+        if crate::plugin::try_dispatch(self, i, cmd) {
+            return Task::none();
+        }
+
         match cmd {
-            // ── Storm Sewer module ──────────────────────────────────────────
-            "SS_ANALYZE" => {
-                use crate::modules::storm_sewer::analysis as ss;
-                let result = ss::analyze_doc(self.tabs[i].scene.document.entities());
-                match result {
-                    Ok((ents, report)) => {
-                        for e in ents {
-                            let _ = self.tabs[i].scene.add_entity(e);
-                        }
-                        self.tabs[i].scene.bump_geometry();
-                        self.command_line
-                            .push_info("Storm sewer: analyzed drawn network (default IDF 60/(t+10)^0.8).");
-                        for line in report.lines() {
-                            self.command_line.push_output(line);
-                        }
-                    }
-                    Err(e) => self.command_line.push_error(&e),
-                }
-            }
-            "SS_REPORT" => {
-                use crate::modules::storm_sewer::analysis as ss;
-                match ss::report_doc(self.tabs[i].scene.document.entities()) {
-                    Ok(report) => {
-                        for line in report.lines() {
-                            self.command_line.push_output(line);
-                        }
-                    }
-                    Err(e) => self.command_line.push_error(&e),
-                }
-            }
-            "SS_INLET" | "SS_JUNCTION" | "SS_OUTFALL" => {
-                use crate::modules::storm_sewer::structures::PlaceStructure;
-                let cmd = match cmd {
-                    "SS_INLET" => PlaceStructure::inlet(),
-                    "SS_JUNCTION" => PlaceStructure::junction(),
-                    _ => PlaceStructure::outfall(),
-                };
-                self.command_line.push_info(&cmd.prompt());
-                self.tabs[i].active_cmd = Some(Box::new(cmd));
-            }
-            "SS_PIPE" => {
-                let cmd = crate::modules::storm_sewer::structures::PlacePipe::new();
-                self.command_line.push_info(&cmd.prompt());
-                self.tabs[i].active_cmd = Some(Box::new(cmd));
-            }
-            "SS_PROFILE" => {
-                use crate::modules::storm_sewer::analysis as ss;
-                let result = ss::profile_doc(self.tabs[i].scene.document.entities());
-                match result {
-                    Ok(ents) => {
-                        for e in ents {
-                            let _ = self.tabs[i].scene.add_entity(e);
-                        }
-                        self.tabs[i].scene.bump_geometry();
-                        self.command_line.push_info("Storm sewer HGL profile drawn.");
-                    }
-                    Err(e) => self.command_line.push_error(&e),
-                }
-            }
             "NEW" => return Task::done(Message::TabNew),
             "OPEN" => return Task::done(Message::OpenFile),
             "SAVE" | "QSAVE" => return Task::done(Message::SaveFile),

src/app/document.rs

@@ -9,6 +9,8 @@ use crate::ui::{LayerPanel, PropertiesPanel};
 use acadrust::tables::Ucs;
 use acadrust::{CadDocument, Handle};
 use iced;
+use std::any::{Any, TypeId};
+use std::collections::HashMap;
 use std::path::PathBuf;
 
 // ── Dynamic input ──────────────────────────────────────────────────────────
@@ -96,6 +98,8 @@ pub(super) struct DocumentTab {
     /// of the model-space shader. The scene is still constructed so the
     /// rest of the code can treat it as a normal tab when reading.
     pub(super) is_start: bool,
+    /// Per-plugin document state (`plugin::BuiltinPlugin` manifest id → state).
+    pub(super) plugin_state: HashMap<&'static str, Box<dyn Any + Send + Sync>>,
 }
 
 impl DocumentTab {
@@ -142,9 +146,41 @@ impl DocumentTab {
             active_mleader_style: "Standard".to_string(),
             last_synced_camera_gen: 0,
             is_start: false,
+            plugin_state: HashMap::new(),
         }
     }
 
+    pub(super) fn plugin_state<T: Any + Send + Sync + 'static>(
+        &self,
+        plugin_id: &'static str,
+        _type_id: TypeId,
+    ) -> Option<&T> {
+        self.plugin_state.get(plugin_id)?.downcast_ref::<T>()
+    }
+
+    pub(super) fn plugin_state_mut<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+        _type_id: TypeId,
+    ) -> Option<&mut T> {
+        self.plugin_state.get_mut(plugin_id)?.downcast_mut::<T>()
+    }
+
+    pub(super) fn ensure_plugin_state<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+        init: impl FnOnce() -> T,
+    ) -> &mut T {
+        if !self.plugin_state.contains_key(plugin_id) {
+            self.plugin_state.insert(plugin_id, Box::new(init()));
+        }
+        self.plugin_state
+            .get_mut(plugin_id)
+            .expect("just inserted")
+            .downcast_mut::<T>()
+            .expect("plugin_id type mismatch")
+    }
+
     /// Welcome / Start tab. Carries a dummy Scene so the rest of the app
     /// can read tab state uniformly; the viewport renderer detects
     /// `is_start` and shows a welcome page instead.

src/app/mod.rs

@@ -1,5 +1,6 @@
 mod cmd_result;
 mod commands;
+pub mod plugin_host;
 mod document;
 mod helpers;
 mod history;

src/app/plugin_host.rs

@@ -0,0 +1,94 @@
+// HostSession — plugin-facing API implemented inside `app` (private field access).
+
+use std::any::{Any, TypeId};
+
+use acadrust::{CadDocument, EntityType, Handle};
+
+use super::OpenCADStudio;
+use crate::command::CadCommand;
+
+/// Session adapter: one active document tab, command line, undo.
+pub(crate) struct HostSession<'a> {
+    app: &'a mut OpenCADStudio,
+    tab: usize,
+}
+
+impl<'a> HostSession<'a> {
+    pub(crate) fn new(app: &'a mut OpenCADStudio, tab: usize) -> Self {
+        Self { app, tab }
+    }
+
+    pub fn tab_index(&self) -> usize {
+        self.tab
+    }
+
+    pub fn document(&self) -> &CadDocument {
+        &self.app.tabs[self.tab].scene.document
+    }
+
+    pub fn document_mut(&mut self) -> &mut CadDocument {
+        &mut self.app.tabs[self.tab].scene.document
+    }
+
+    pub fn entities(&self) -> impl Iterator<Item = &EntityType> {
+        self.document().entities()
+    }
+
+    pub fn entities_mut(&mut self) -> impl Iterator<Item = &mut EntityType> {
+        self.document_mut().entities_mut()
+    }
+
+    pub fn add_entity(&mut self, entity: EntityType) -> Handle {
+        self.app.tabs[self.tab].scene.add_entity(entity)
+    }
+
+    pub fn bump_geometry(&mut self) {
+        self.app.tabs[self.tab].scene.bump_geometry();
+    }
+
+    pub fn push_undo(&mut self, label: &str) {
+        self.app.push_undo_snapshot(self.tab, label);
+    }
+
+    pub fn set_dirty(&mut self) {
+        self.app.tabs[self.tab].dirty = true;
+    }
+
+    pub fn push_info(&mut self, msg: &str) {
+        self.app.command_line.push_info(msg);
+    }
+
+    pub fn push_output(&mut self, msg: &str) {
+        self.app.command_line.push_output(msg);
+    }
+
+    pub fn push_error(&mut self, msg: &str) {
+        self.app.command_line.push_error(msg);
+    }
+
+    pub fn set_active_command(&mut self, cmd: Box<dyn CadCommand>) {
+        self.app.tabs[self.tab].active_cmd = Some(cmd);
+    }
+
+    pub fn plugin_state<T: Any + Send + Sync + 'static>(
+        &self,
+        plugin_id: &'static str,
+    ) -> Option<&T> {
+        self.app.tabs[self.tab].plugin_state(plugin_id, TypeId::of::<T>())
+    }
+
+    pub fn plugin_state_mut<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+    ) -> Option<&mut T> {
+        self.app.tabs[self.tab].plugin_state_mut(plugin_id, TypeId::of::<T>())
+    }
+
+    pub fn ensure_plugin_state<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+        init: impl FnOnce() -> T,
+    ) -> &mut T {
+        self.app.tabs[self.tab].ensure_plugin_state(plugin_id, init)
+    }
+}

src/app/update.rs

@@ -2278,6 +2278,11 @@ impl OpenCADStudio {
                         .as_ref()
                         .map(|c| c.needs_entity_pick())
                         .unwrap_or(false);
+                    let needs_structure = self.tabs[i]
+                        .active_cmd
+                        .as_ref()
+                        .map(|c| c.needs_structure_point_pick())
+                        .unwrap_or(false);
                     let is_gathering = self.tabs[i]
                         .active_cmd
                         .as_ref()
@@ -2288,7 +2293,7 @@ impl OpenCADStudio {
                         .as_ref()
                         .map(|c| c.needs_tangent_pick())
                         .unwrap_or(false);
-                    self.tabs[i].snap_result = if needs_entity || is_gathering {
+                    self.tabs[i].snap_result = if needs_entity || is_gathering || needs_structure {
                         None
                     } else if needs_tan {
                         self.snapper.snap_tangent_only(
@@ -2345,6 +2350,39 @@ impl OpenCADStudio {
                     };
                     self.tabs[i].last_cursor_world = effective;
                     self.tabs[i].last_cursor_screen = p_full;
+
+                    // C3D-style orange object snap (plugin commands implement resolve_object_pick).
+                    if needs_structure {
+                        use crate::snap::{SnapResult, SnapType};
+                        let pick = self.tabs[i].active_cmd.as_ref().and_then(|c| {
+                            c.resolve_object_pick(
+                                &self.tabs[i].scene,
+                                effective.x as f64,
+                                effective.y as f64,
+                            )
+                        });
+                        if let Some(pick) = pick {
+                            let world =
+                                glam::Vec3::new(pick.x as f32, pick.y as f32, effective.z);
+                            let ndc = view_proj.project_point3(world);
+                            let screen = iced::Point::new(
+                                (ndc.x + 1.0) * 0.5 * bounds.width,
+                                (1.0 - ndc.y) * 0.5 * bounds.height,
+                            );
+                            self.tabs[i].snap_result = Some(SnapResult {
+                                world,
+                                screen,
+                                snap_type: SnapType::ObjectPick,
+                                tangent_obj: None,
+                            });
+                            if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
+                                cmd.set_acquisition_hint(Some(pick.label));
+                            }
+                        } else if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
+                            cmd.set_acquisition_hint(None);
+                        }
+                    }
+
                     // Snap glyph is positioned in canvas space; shift the
                     // tile-local snap screen point back to the full canvas.
                     if let Some(s) = self.tabs[i].snap_result.as_mut() {
@@ -2352,16 +2390,42 @@ impl OpenCADStudio {
                         s.screen.y += tile_b.y;
                     }
 
-                    let mut previews = if needs_entity {
+                    let mut previews = if needs_structure {
+                        let mut p = self.tabs[i]
+                            .active_cmd
+                            .as_ref()
+                            .map(|c| c.object_pick_hover_previews(&self.tabs[i].scene, effective))
+                            .unwrap_or_default();
+                        if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
+                            p.extend(cmd.on_preview_wires(effective));
+                        }
+                        p
+                    } else if needs_entity {
                         let hover_handle =
                             scene::hit_test::click_hit(p, &all_wires[..], view_proj, bounds)
                                 .and_then(|s| Scene::handle_from_wire_name(s))
                                 .unwrap_or(acadrust::Handle::NULL);
-                        self.tabs[i]
+                        let mut p = self.tabs[i]
                             .active_cmd
                             .as_mut()
                             .map(|c| c.on_hover_entity(hover_handle, effective))
-                            .unwrap_or_default()
+                            .unwrap_or_default();
+                        if !hover_handle.is_null() {
+                            if let Some(cmd) = self.tabs[i].active_cmd.as_ref() {
+                                p.extend(cmd.entity_pick_acquire_previews(
+                                    &self.tabs[i].scene,
+                                    hover_handle,
+                                ));
+                            }
+                            if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
+                                if let Some(hint) =
+                                    cmd.entity_pick_acquire_hint(hover_handle)
+                                {
+                                    cmd.set_acquisition_hint(Some(hint));
+                                }
+                            }
+                        }
+                        p
                     } else {
                         self.tabs[i]
                             .active_cmd
@@ -2763,6 +2827,36 @@ impl OpenCADStudio {
                     };
 
                     let result = if self.tabs[i]
+                        .active_cmd
+                        .as_ref()
+                        .map(|c| c.needs_structure_point_pick())
+                        .unwrap_or(false)
+                    {
+                        let pick = self.tabs[i].active_cmd.as_ref().and_then(|c| {
+                            c.resolve_object_pick(
+                                &self.tabs[i].scene,
+                                world_pt.x as f64,
+                                world_pt.y as f64,
+                            )
+                        });
+                        if let Some(pick) = pick {
+                            let center = glam::Vec3::new(pick.x as f32, pick.y as f32, world_pt.z);
+                            let result = self.tabs[i].active_cmd.as_mut().map(|c| {
+                                c.on_structure_pick(pick.handle, center)
+                            });
+                            self.command_line
+                                .push_info(&format!("{} acquired.", pick.label));
+                            result
+                        } else {
+                            let msg = self.tabs[i]
+                                .active_cmd
+                                .as_ref()
+                                .map(|c| c.object_pick_miss_message())
+                                .unwrap_or("No object near click.");
+                            self.command_line.push_error(msg);
+                            None
+                        }
+                    } else if self.tabs[i]
                         .active_cmd
                         .as_ref()
                         .map(|c| c.needs_entity_pick())
@@ -2773,6 +2867,23 @@ impl OpenCADStudio {
                         let hit = scene::hit_test::click_hit(p, &all_wires2[..], vp_mat2, bounds)
                             .and_then(|s| Scene::handle_from_wire_name(s));
                         if let Some(handle) = hit {
+                            // Some commands (e.g. SS_CATCHMENT) need the entity
+                            // body before `on_entity_pick` can advance.
+                            let inject_first = self.tabs[i]
+                                .active_cmd
+                                .as_ref()
+                                .map(|c| c.inject_before_entity_pick())
+                                .unwrap_or(false);
+                            if inject_first {
+                                if let Some(entity) =
+                                    self.tabs[i].scene.document.get_entity(handle).cloned()
+                                {
+                                    if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
+                                        cmd.inject_picked_entity(entity);
+                                    }
+                                }
+                            }
+
                             let result = self.tabs[i]
                                 .active_cmd
                                 .as_mut()

src/command/mod.rs

@@ -7,9 +7,19 @@
 
 use crate::scene::hatch_model::HatchModel;
 use crate::scene::wire_model::WireModel;
+use crate::scene::Scene;
 use acadrust::{EntityType, Handle};
 use glam::Vec3;
 
+/// Domain object resolved under the cursor for C3D-style ObjectPick snapping.
+#[derive(Clone, Copy, Debug)]
+pub struct ObjectPickHit {
+    pub handle: Handle,
+    pub x: f64,
+    pub y: f64,
+    pub label: &'static str,
+}
+
 // ── Transform ─────────────────────────────────────────────────────────────
 
 /// A geometric transformation applied to existing entities.
@@ -267,6 +277,44 @@ pub trait CadCommand: Send {
         CmdResult::Cancel
     }
 
+    /// Point-click pick of domain objects (wire hit-test often misses small markers).
+    fn needs_structure_point_pick(&self) -> bool {
+        false
+    }
+
+    /// Resolve a domain object near `(x, y)` while `needs_structure_point_pick()` is active.
+    fn resolve_object_pick(&self, _scene: &Scene, _x: f64, _y: f64) -> Option<ObjectPickHit> {
+        None
+    }
+
+    /// Preview wires while hovering during object-point pick.
+    fn object_pick_hover_previews(&self, _scene: &Scene, _cursor: Vec3) -> Vec<WireModel> {
+        vec![]
+    }
+
+    /// Message when `resolve_object_pick` returns none on click.
+    fn object_pick_miss_message(&self) -> &'static str {
+        "No object near click."
+    }
+
+    /// Called when `needs_structure_point_pick()` is true and a structure is found near the click.
+    fn on_structure_pick(&mut self, _handle: Handle, _pt: Vec3) -> CmdResult {
+        CmdResult::Cancel
+    }
+
+    /// Extra acquisition previews during entity pick (besides `on_hover_entity`).
+    fn entity_pick_acquire_previews(&self, _scene: &Scene, _handle: Handle) -> Vec<WireModel> {
+        vec![]
+    }
+
+    /// Acquisition hint label during entity pick hover.
+    fn entity_pick_acquire_hint(&self, _handle: Handle) -> Option<&'static str> {
+        None
+    }
+
+    /// Hover label for C3D-style object acquisition (e.g. "Inlet" under cursor).
+    fn set_acquisition_hint(&mut self, _hint: Option<&str>) {}
+
     /// Called after `CmdResult::ReplaceEntity` is applied to the document.
     /// `old` is the erased handle; `new_handles` are the handles assigned to the replacement entities.
     /// Commands that stay active across replaces should update their internal snapshots here.
@@ -367,7 +415,13 @@ pub trait CadCommand: Send {
         self.on_point(hit)
     }
 
-    /// Called by update.rs after `on_entity_pick` to inject the cloned entity into commands
+    /// When true, `update.rs` injects the picked entity before calling
+    /// `on_entity_pick` (required when the pick handler reads injected state).
+    fn inject_before_entity_pick(&self) -> bool {
+        false
+    }
+
+    /// Called by update.rs to inject the cloned entity into commands
     /// that need to read/modify it (e.g. DIMTEDIT, MLEADERADD, MLEADERREMOVE).
     /// Default: no-op.
     fn inject_picked_entity(&mut self, _entity: acadrust::EntityType) {}

src/lib.rs

@@ -6,6 +6,7 @@ pub mod entities;
 pub mod io;
 pub mod linetypes;
 pub mod modules;
+pub mod plugin;
 pub mod patterns;
 pub mod scene;
 pub mod snap;

src/main.rs

@@ -10,6 +10,7 @@ mod entities;
 mod io;
 mod linetypes;
 mod modules;
+mod plugin;
 mod patterns;
 mod scene;
 mod snap;

src/modules/mod.rs

@@ -1,10 +1,13 @@
 // Module system — CadModule, ToolDef, RibbonGroup.
 //
-// To add a new module:
-//   1. Create `src/modules/my_name/` directory
+// To add a **core** ribbon tab (Home, View, …):
+//   1. Create `src/modules/my_name/` directory (no `plugin.toml`)
 //   2. Add `src/modules/my_name/mod.rs` implementing `CadModule` as `MyNameModule`
 //   3. Add `pub mod my_name;` below
-//   4. `cargo build` — module appears in the ribbon automatically
+//   4. `cargo build` — tab appears via build.rs registry
+//
+// To add an **add-on plugin** (Storm Sewer, …):
+//   See `docs/plugin-architecture.md` and copy `docs/plugin-template/`.
 //
 // Each module folder contains:
 //   - mod.rs       : module definition (ribbon groups + tool layout)

src/modules/registry.rs

@@ -14,6 +14,5 @@ pub fn all_modules() -> Vec<Box<dyn CadModule>> {
 		Box::new(super::view::ViewModule),
 		Box::new(super::manage::ManageModule),
 		Box::new(super::layout::LayoutModule),
-		Box::new(super::storm_sewer::StormSewerModule),
 	]
 }

src/modules/storm_sewer/PLUGIN.md

@@ -0,0 +1,69 @@
+# Storm Sewer (`opencad.storm_sewer`)
+
+Add-on package for gravity storm-drain network design and analysis.
+
+- **Engine:** `crates/stormsewer` (headless hydraulics)
+- **Host integration:** `plugin.rs`, `dispatch.rs`, `register.rs`
+- **Architecture:** `docs/plugin-architecture.md`
+
+## Commands
+
+| Command | Description |
+|---------|-------------|
+| `SS_INLET` | Place inlet structure |
+| `SS_JUNCTION` | Place junction structure |
+| `SS_OUTFALL` | Place outfall structure |
+| `SS_PIPE` | Draw pipe between two structures |
+| `SS_CATCHMENT` | Draw catchment boundary polyline |
+| `SS_APPLYTC` | Apply time-of-concentration from catchments |
+| `SS_LANDXML` / `SS_IMPORTXML` | Import LandXML storm network |
+| `SS_ANALYZE` | Run hydraulic analysis on drawn network |
+| `SS_SIZE` | Size pipes from analysis |
+| `SS_PARAMS` | Set rainfall / analysis parameters |
+| `SS_MULTIRP` | Multi return-period analysis |
+| `SS_REPORT` | Print analysis report |
+| `SS_PROFILE` | Draw HGL profile |
+
+## XDATA schemas
+
+All records use application names registered in `manifest.rs`. Values are stored as XDATA on DWG entities so networks round-trip through save/load.
+
+### `STORMSEWER_STRUCT` (on `CIRCLE`)
+
+| Index | Field | Type | Notes |
+|-------|-------|------|-------|
+| 0 | kind | int | 0=inlet, 1=junction, 2=outfall |
+| 1 | invert | real | Structure invert elevation |
+| 2 | rim | real | Rim elevation |
+| 3 | area | real | Contributing area (acres) |
+| 4 | C | real | Runoff coefficient |
+
+### `STORMSEWER_PIPE` (on `LINE`)
+
+| Index | Field | Type | Notes |
+|-------|-------|------|-------|
+| 0 | diameter | real | Pipe diameter (inches) |
+| 1 | n | real | Manning's n |
+| 2 | from_handle | int | Start structure entity handle |
+| 3 | to_handle | int | End structure entity handle |
+
+### `STORMSEWER_CATCHMENT` (on `LWPOLYLINE`)
+
+| Index | Field | Type | Notes |
+|-------|-------|------|-------|
+| 0 | area_acres | real | Catchment area |
+| 1 | C | real | Runoff coefficient |
+| 2 | length_ft | real | Flow path length |
+| 3 | slope | real | Average slope |
+| 4 | inlet_handle | int | Optional inlet structure handle (0 = none) |
+
+## Per-document state
+
+Stored under plugin id `opencad.storm_sewer` as `StormTabState` (`state.rs`):
+
+- `StormAnalysisParams` — IDF, tailwater, min Tc, return periods, etc.
+
+## Dependencies
+
+- `stormsewer` workspace crate — must not depend on CAD host crates.
+- This package — must not edit `src/app/commands.rs`; use `dispatch.rs`.

src/modules/storm_sewer/analysis.rs

@@ -8,18 +8,19 @@
 use acadrust::types::Vector3;
 use acadrust::{Circle, EntityType, Line, MText};
 
+use stormsewer::design::check_inlet;
 use stormsewer::drawing::{draw_network, DrawConfig};
 use stormsewer::idf::IdfCurve;
-use stormsewer::network::{Analysis, AnalysisOptions, Network, Node, Pipe};
+use stormsewer::network::{Analysis, AnalysisOptions, Network, Node, NodeKind, Pipe};
+use stormsewer::params::StormAnalysisParams;
 use stormsewer::parse::parse_ssn;
-use stormsewer::report::format_analysis;
+use stormsewer::report::{format_analysis, format_multi_rp};
 
 use super::data;
 
-/// Default network-level analysis parameters used when analyzing a drawn
-/// network (per-entity data carries geometry/area; rainfall is global).
-fn default_params() -> (IdfCurve, AnalysisOptions) {
-    (IdfCurve::new(60.0, 10.0, 0.8), AnalysisOptions::default())
+/// Default network-level analysis parameters (tests / fallbacks).
+pub fn default_params() -> StormAnalysisParams {
+    StormAnalysisParams::municipal()
 }
 
 /// Annotation labels only (flow + HGL), for overlaying on an already-drawn
@@ -36,27 +37,85 @@ fn build_annotations(net: &Network, a: &Analysis) -> Vec<EntityType> {
 
 /// Reconstruct the network from drawn entities, analyze it, and return
 /// annotation entities (flow/HGL labels) + the report.
-pub fn analyze_doc<'a>(entities: impl Iterator<Item = &'a EntityType>) -> Result<(Vec<EntityType>, String), String> {
+pub fn analyze_doc<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    params: &StormAnalysisParams,
+) -> Result<(Vec<EntityType>, String, Analysis), String> {
     let net = data::network_from_entities(entities)?;
-    let (idf, opts) = default_params();
-    let a = run_analysis(&net, &idf, &opts)?;
-    Ok((build_annotations(&net, &a), format_analysis(&a)))
+    let a = run_analysis(&net, params.idf.design_curve(), &params.hydraulics)?;
+    let report = full_report(&net, &a, params);
+    Ok((build_annotations(&net, &a), report, a))
 }
 
 /// Reconstruct from drawn entities and return the HGL long-section entities.
-pub fn profile_doc<'a>(entities: impl Iterator<Item = &'a EntityType>) -> Result<Vec<EntityType>, String> {
+pub fn profile_doc<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    params: &StormAnalysisParams,
+) -> Result<Vec<EntityType>, String> {
     let net = data::network_from_entities(entities)?;
-    let (idf, opts) = default_params();
-    let a = run_analysis(&net, &idf, &opts)?;
+    let a = run_analysis(&net, params.idf.design_curve(), &params.hydraulics)?;
     Ok(build_profile(&net, &a))
 }
 
 /// Reconstruct from drawn entities and return the formatted report.
-pub fn report_doc<'a>(entities: impl Iterator<Item = &'a EntityType>) -> Result<String, String> {
+pub fn report_doc<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    params: &StormAnalysisParams,
+) -> Result<String, String> {
     let net = data::network_from_entities(entities)?;
-    let (idf, opts) = default_params();
-    let a = run_analysis(&net, &idf, &opts)?;
-    Ok(format_analysis(&a))
+    let a = run_analysis(&net, params.idf.design_curve(), &params.hydraulics)?;
+    Ok(full_report(&net, &a, params))
+}
+
+/// Multi-return-period peak-flow comparison table.
+pub fn multi_rp_report<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    params: &StormAnalysisParams,
+) -> Result<String, String> {
+    let net = data::network_from_entities(entities)?;
+    Ok(format_multi_rp(&net, &params.idf, &params.hydraulics))
+}
+
+fn full_report(net: &Network, a: &Analysis, params: &StormAnalysisParams) -> String {
+    let mut s = format_analysis(a);
+    s.push_str(&inlet_section(net, a, params));
+    s
+}
+
+fn inlet_section(net: &Network, a: &Analysis, params: &StormAnalysisParams) -> String {
+    let mut s = String::new();
+    let mut any = false;
+    for nd in &net.nodes {
+        if nd.kind != NodeKind::Inlet {
+            continue;
+        }
+        let q = a
+            .pipes
+            .iter()
+            .filter(|p| p.from == nd.id)
+            .map(|p| p.design_q)
+            .fold(0.0f64, f64::max);
+        if q <= 0.0 {
+            continue;
+        }
+        if !any {
+            s.push_str("\n=== INLET CAPACITY (HEC-22 grate, simplified) ===\n");
+            s.push_str("Node   Q(cfs)  Cap(cfs)  Status\n");
+            any = true;
+        }
+        let chk = check_inlet(
+            q,
+            params.inlet_grate_length_ft,
+            params.inlet_flow_depth_ft,
+            params.inlet_gutter_slope,
+        );
+        let status = if chk.ok { "ok" } else { "BYPASS" };
+        s.push_str(&format!(
+            "{:<6} {:>6.2} {:>8.2}  {status}\n",
+            nd.id, chk.design_q_cfs, chk.capacity_cfs
+        ));
+    }
+    s
 }
 
 /// The built-in demonstration network (properly sized, with plan coordinates).

src/modules/storm_sewer/catchment.rs

@@ -0,0 +1,309 @@
+// Interactive catchment tagging: closed LwPolyline + STORMSEWER_CATCHMENT XDATA.
+
+use acadrust::entities::LwPolyline;
+use acadrust::{EntityType, Handle};
+use glam::Vec3;
+
+use stormsewer::catchment::{default_flow_length_ft, polygon_centroid, shoelace_area_sqft, sqft_to_acres};
+
+use super::data;
+use super::preview;
+use crate::command::{CadCommand, CmdResult, ObjectPickHit};
+use crate::scene::{Scene, WireModel};
+
+fn parse_num(text: &str) -> Option<f64> {
+    text.trim().replace(',', ".").parse::<f64>().ok()
+}
+
+enum CStep {
+    PickPolyline,
+    RunoffC,
+    FlowLength,
+    Slope,
+    PickInlet,
+}
+
+pub struct TagCatchment {
+    step: CStep,
+    picked: Option<EntityType>,
+    c: f64,
+    flow_length: f64,
+    slope: f64,
+    inlet_handle: Handle,
+    area_ac: f64,
+    acquire_hint: Option<String>,
+}
+
+impl TagCatchment {
+    pub fn new() -> Self {
+        Self {
+            step: CStep::PickPolyline,
+            picked: None,
+            c: 0.70,
+            flow_length: 0.0,
+            slope: 0.01,
+            inlet_handle: Handle::NULL,
+            area_ac: 0.0,
+            acquire_hint: None,
+        }
+    }
+
+    fn commit(&self) -> CmdResult {
+        let mut ent = self.picked.clone().expect("polyline not picked");
+        let EntityType::LwPolyline(pl) = &ent else {
+            return CmdResult::Cancel;
+        };
+        if !pl.is_closed {
+            return CmdResult::Cancel;
+        }
+        let handle = ent.common().handle;
+        let xd = &mut ent.common_mut().extended_data;
+        let kept: Vec<_> = xd
+            .records()
+            .iter()
+            .filter(|r| r.application_name != data::APP_CATCHMENT)
+            .cloned()
+            .collect();
+        xd.clear();
+        for r in kept {
+            xd.add_record(r);
+        }
+        xd.add_record(data::catchment_xdata(
+            self.c,
+            self.flow_length,
+            self.slope,
+            self.inlet_handle,
+        ));
+        CmdResult::ReplaceMany(vec![(handle, vec![ent])], vec![])
+    }
+
+    fn assign_inlet(&mut self, handle: Handle, pt: Vec3) {
+        self.inlet_handle = handle;
+        if self.flow_length <= 0.0 {
+            if let Some(ref ent) = self.picked {
+                if let EntityType::LwPolyline(pl) = ent {
+                    let verts: Vec<_> = pl.vertices.iter().map(|v| (v.location.x, v.location.y)).collect();
+                    let centroid = polygon_centroid(&verts);
+                    self.flow_length =
+                        default_flow_length_ft(centroid, (pt.x as f64, pt.y as f64));
+                }
+            }
+        }
+    }
+}
+
+impl Default for TagCatchment {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl CadCommand for TagCatchment {
+    fn name(&self) -> &'static str {
+        "SS_CATCHMENT"
+    }
+
+    fn prompt(&self) -> String {
+        match self.step {
+            CStep::PickPolyline => {
+                "Catchment: click closed drainage-area polyline (highlights orange):".into()
+            }
+            CStep::RunoffC => format!(
+                "Catchment area {:.3} ac — runoff C <{:.2}> (Enter to accept):",
+                self.area_ac, self.c
+            ),
+            CStep::FlowLength => format!(
+                "Flow path length, ft <{:.1}> (0 = auto from centroid to inlet):",
+                self.flow_length
+            ),
+            CStep::Slope => format!("Average slope, ft/ft <{:.4}> (Enter to accept):", self.slope),
+            CStep::PickInlet => {
+                let hint = self
+                    .acquire_hint
+                    .as_deref()
+                    .map(|h| format!(" [{h}]"))
+                    .unwrap_or_default();
+                format!(
+                    "Catchment: click inlet/junction to drain to (orange snap){hint} — Enter = nearest:"
+                )
+            }
+        }
+    }
+
+    fn set_acquisition_hint(&mut self, hint: Option<&str>) {
+        if matches!(self.step, CStep::PickInlet) {
+            self.acquire_hint = hint.map(str::to_string);
+        }
+    }
+
+    fn wants_text_input(&self) -> bool {
+        matches!(self.step, CStep::RunoffC | CStep::FlowLength | CStep::Slope)
+    }
+
+    fn needs_entity_pick(&self) -> bool {
+        matches!(self.step, CStep::PickPolyline)
+    }
+
+    fn needs_structure_point_pick(&self) -> bool {
+        matches!(self.step, CStep::PickInlet)
+    }
+
+    fn resolve_object_pick(&self, scene: &Scene, x: f64, y: f64) -> Option<ObjectPickHit> {
+        let pick = preview::structure_under_cursor(scene, x, y, true)?;
+        Some(ObjectPickHit {
+            handle: pick.handle,
+            x: pick.x,
+            y: pick.y,
+            label: pick.label(),
+        })
+    }
+
+    fn object_pick_hover_previews(&self, scene: &Scene, cursor: Vec3) -> Vec<WireModel> {
+        preview::structure_acquire_previews(scene, cursor, true)
+    }
+
+    fn object_pick_miss_message(&self) -> &'static str {
+        "No storm structure near click — move closer or press Enter for nearest."
+    }
+
+    fn entity_pick_acquire_previews(&self, scene: &Scene, handle: Handle) -> Vec<WireModel> {
+        if matches!(self.step, CStep::PickPolyline) {
+            preview::catchment_poly_under_cursor(scene, handle)
+        } else {
+            vec![]
+        }
+    }
+
+    fn entity_pick_acquire_hint(&self, _handle: Handle) -> Option<&'static str> {
+        if matches!(self.step, CStep::PickPolyline) {
+            Some("Catchment area")
+        } else {
+            None
+        }
+    }
+
+    fn inject_picked_entity(&mut self, entity: EntityType) {
+        if !matches!(self.step, CStep::PickPolyline) {
+            return;
+        }
+        if let EntityType::LwPolyline(ref pl) = entity {
+            if pl.is_closed && pl.vertices.len() >= 3 {
+                let verts: Vec<_> = pl.vertices.iter().map(|v| (v.location.x, v.location.y)).collect();
+                self.area_ac = sqft_to_acres(shoelace_area_sqft(&verts));
+                self.picked = Some(entity);
+            }
+        }
+    }
+
+    fn inject_before_entity_pick(&self) -> bool {
+        true
+    }
+
+    fn on_entity_pick(&mut self, _handle: Handle, _pt: Vec3) -> CmdResult {
+        if self.picked.is_none() {
+            return CmdResult::NeedPoint;
+        }
+        self.step = CStep::RunoffC;
+        CmdResult::NeedPoint
+    }
+
+    fn on_structure_pick(&mut self, handle: Handle, pt: Vec3) -> CmdResult {
+        self.assign_inlet(handle, pt);
+        self.acquire_hint = None;
+        self.commit()
+    }
+
+    fn on_text_input(&mut self, text: &str) -> Option<CmdResult> {
+        let v = parse_num(text);
+        match self.step {
+            CStep::RunoffC => {
+                if let Some(x) = v {
+                    self.c = x;
+                }
+                self.step = CStep::FlowLength;
+            }
+            CStep::FlowLength => {
+                if let Some(x) = v {
+                    self.flow_length = x;
+                }
+                self.step = CStep::Slope;
+            }
+            CStep::Slope => {
+                if let Some(x) = v {
+                    self.slope = x;
+                }
+                self.step = CStep::PickInlet;
+            }
+            _ => {}
+        }
+        None
+    }
+
+    fn on_enter(&mut self) -> CmdResult {
+        match self.step {
+            CStep::PickInlet => {
+                self.acquire_hint = None;
+                self.commit()
+            }
+            CStep::RunoffC => {
+                self.step = CStep::FlowLength;
+                CmdResult::NeedPoint
+            }
+            CStep::FlowLength => {
+                self.step = CStep::Slope;
+                CmdResult::NeedPoint
+            }
+            CStep::Slope => {
+                self.step = CStep::PickInlet;
+                CmdResult::NeedPoint
+            }
+            CStep::PickPolyline => CmdResult::NeedPoint,
+        }
+    }
+
+    fn on_point(&mut self, _pt: Vec3) -> CmdResult {
+        CmdResult::NeedPoint
+    }
+}
+
+/// Inspect a closed polyline and suggest defaults from geometry.
+pub fn catchment_defaults_from_poly(pl: &LwPolyline, target_xy: Option<(f64, f64)>) -> (f64, f64, f64) {
+    let verts: Vec<_> = pl.vertices.iter().map(|v| (v.location.x, v.location.y)).collect();
+    let area_ac = sqft_to_acres(shoelace_area_sqft(&verts));
+    let centroid = polygon_centroid(&verts);
+    let flow_len = target_xy
+        .map(|t| default_flow_length_ft(centroid, t))
+        .unwrap_or(0.0);
+    (area_ac, flow_len, 0.01)
+}
+
+/// Update structure entities with Tc computed from catchments + network assembly.
+pub fn apply_tc_from_network<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    entities_mut: impl Iterator<Item = &'a mut EntityType>,
+) -> Result<usize, String> {
+    data::apply_tc_in_document(entities, entities_mut)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use acadrust::entities::LwVertex;
+    use acadrust::types::Vector2;
+
+    #[test]
+    fn defaults_from_unit_square() {
+        let mut pl = LwPolyline::default();
+        pl.is_closed = true;
+        pl.vertices = vec![
+            LwVertex::new(Vector2::new(0.0, 0.0)),
+            LwVertex::new(Vector2::new(10.0, 0.0)),
+            LwVertex::new(Vector2::new(10.0, 10.0)),
+            LwVertex::new(Vector2::new(0.0, 10.0)),
+        ];
+        let (area, flow, slope) = catchment_defaults_from_poly(&pl, Some((0.0, 0.0)));
+        assert!((area - sqft_to_acres(100.0)).abs() < 1e-6);
+        assert!(flow > 0.0);
+        assert!((slope - 0.01).abs() < 1e-9);
+    }
+}

src/modules/storm_sewer/data.rs

@@ -2,19 +2,36 @@
 // of a `stormsewer::Network` from the drawn entities.
 //
 // Structures are circles tagged with the `STORMSEWER_STRUCT` app record
-// [kind, invert, rim, area, C]; pipes are lines tagged with `STORMSEWER_PIPE`
-// [diameter, n, from-handle, to-handle]. Connectivity is by entity handle, so
-// the drawn network round-trips to DWG/DXF and is analyzable directly.
+// [kind, invert, rim, area, C, tc_inlet?]; pipes are lines tagged with
+// `STORMSEWER_PIPE` [diameter, n, from-handle, to-handle]. Catchments are
+// closed LwPolylines tagged with `STORMSEWER_CATCHMENT`
+// [C, flow_length_ft, slope, inlet_handle (0 = auto)]. Connectivity is by
+// entity handle, so the drawn network round-trips to DWG/DXF and is analyzable
+// directly.
 
 use std::collections::HashMap;
 
+use acadrust::entities::LwPolyline;
 use acadrust::xdata::{ExtendedDataRecord, XDataValue};
 use acadrust::{EntityType, Handle};
 
+use stormsewer::catchment::{catchment_tc_minutes, default_flow_length_ft, polygon_centroid, shoelace_area_sqft, sqft_to_acres};
 use stormsewer::network::{Network, Node, NodeKind, Pipe};
 
+/// A storm network reconstructed from drawing entities, with entity handles
+/// for round-tripping sizing edits back to the document.
+#[derive(Debug)]
+pub struct DrawnNetwork {
+    pub network: Network,
+    /// Structure entity handles, same order as `network.nodes`.
+    pub node_handles: Vec<Handle>,
+    /// Pipe entity handles, same order as `network.pipes`.
+    pub pipe_handles: Vec<Handle>,
+}
+
 pub const APP_STRUCT: &str = "STORMSEWER_STRUCT";
 pub const APP_PIPE: &str = "STORMSEWER_PIPE";
+pub const APP_CATCHMENT: &str = "STORMSEWER_CATCHMENT";
 
 pub fn kind_str(k: NodeKind) -> &'static str {
     match k {
@@ -34,12 +51,25 @@ fn parse_kind(s: &str) -> NodeKind {
 
 /// XDATA record for a structure marker.
 pub fn structure_xdata(kind: NodeKind, invert: f64, rim: f64, area: f64, c: f64) -> ExtendedDataRecord {
+    structure_xdata_tc(kind, invert, rim, area, c, 10.0)
+}
+
+/// XDATA record for a structure marker including inlet Tc (minutes).
+pub fn structure_xdata_tc(
+    kind: NodeKind,
+    invert: f64,
+    rim: f64,
+    area: f64,
+    c: f64,
+    tc_inlet: f64,
+) -> ExtendedDataRecord {
     let mut r = ExtendedDataRecord::new(APP_STRUCT);
     r.add_value(XDataValue::String(kind_str(kind).to_string()));
     r.add_value(XDataValue::Real(invert));
     r.add_value(XDataValue::Real(rim));
     r.add_value(XDataValue::Real(area));
     r.add_value(XDataValue::Real(c));
+    r.add_value(XDataValue::Real(tc_inlet));
     r
 }
 
@@ -53,6 +83,17 @@ pub fn pipe_xdata(diameter: f64, n: f64, from: Handle, to: Handle) -> ExtendedDa
     r
 }
 
+/// XDATA for a catchment drainage polygon.
+/// `inlet_handle` = `Handle::NULL` for auto-assign to nearest inlet/junction.
+pub fn catchment_xdata(c: f64, flow_length_ft: f64, slope: f64, inlet_handle: Handle) -> ExtendedDataRecord {
+    let mut r = ExtendedDataRecord::new(APP_CATCHMENT);
+    r.add_value(XDataValue::Real(c));
+    r.add_value(XDataValue::Real(flow_length_ft));
+    r.add_value(XDataValue::Real(slope));
+    r.add_value(XDataValue::Handle(inlet_handle));
+    r
+}
+
 fn real(v: &XDataValue) -> Option<f64> {
     if let XDataValue::Real(x) = v {
         Some(*x)
@@ -76,10 +117,12 @@ struct StructRec {
     rim: f64,
     area: f64,
     c: f64,
+    tc_inlet: f64,
     x: f64,
     y: f64,
 }
 
+#[derive(Clone)]
 struct PipeRec {
     diameter: f64,
     n: f64,
@@ -88,6 +131,99 @@ struct PipeRec {
     length: f64,
 }
 
+struct CatchmentRec {
+    c: f64,
+    flow_length_ft: f64,
+    slope: f64,
+    inlet_handle: Option<Handle>,
+    area_ac: f64,
+    centroid: (f64, f64),
+}
+
+/// True when the entity is a tagged storm-sewer structure circle.
+pub fn is_structure_entity(e: &EntityType) -> bool {
+    read_structure(e).is_some()
+}
+
+/// Storm structure resolved from a plan click (center + kind).
+#[derive(Clone, Debug)]
+pub struct StructurePick {
+    pub handle: Handle,
+    pub kind: NodeKind,
+    pub x: f64,
+    pub y: f64,
+}
+
+impl StructurePick {
+    pub fn label(&self) -> &'static str {
+        match self.kind {
+            NodeKind::Inlet => "Inlet",
+            NodeKind::Junction => "Junction",
+            NodeKind::Outfall => "Outfall",
+        }
+    }
+}
+
+/// Nearest storm structure within click tolerance of `(x, y)` (ft).
+/// `pick_padding_ft` is added to each marker circle's radius.
+pub fn structure_at_point<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    x: f64,
+    y: f64,
+    pick_padding_ft: f64,
+    include_outfalls: bool,
+) -> Option<StructurePick> {
+    let mut best: Option<(StructurePick, f64)> = None;
+    for e in entities {
+        let s = read_structure(e)?;
+        if !include_outfalls && s.kind == NodeKind::Outfall {
+            continue;
+        }
+        let radius = match e {
+            EntityType::Circle(c) => c.radius,
+            _ => 3.0,
+        };
+        let dx = s.x - x;
+        let dy = s.y - y;
+        let dist = (dx * dx + dy * dy).sqrt();
+        let limit = radius + pick_padding_ft;
+        if dist <= limit {
+            if best.as_ref().map(|(_, d)| dist < *d).unwrap_or(true) {
+                best = Some((
+                    StructurePick {
+                        handle: s.handle,
+                        kind: s.kind,
+                        x: s.x,
+                        y: s.y,
+                    },
+                    dist,
+                ));
+            }
+        }
+    }
+    best.map(|(p, _)| p)
+}
+
+pub fn nearest_structure_at_point<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    x: f64,
+    y: f64,
+    pick_padding_ft: f64,
+    include_outfalls: bool,
+) -> Option<Handle> {
+    structure_at_point(entities, x, y, pick_padding_ft, include_outfalls).map(|p| p.handle)
+}
+
+/// Nearest inlet/junction only (for catchment drainage targets).
+pub fn nearest_drainage_structure_at_point<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    x: f64,
+    y: f64,
+    pick_padding_ft: f64,
+) -> Option<Handle> {
+    nearest_structure_at_point(entities, x, y, pick_padding_ft, false)
+}
+
 fn read_structure(e: &EntityType) -> Option<StructRec> {
     let rec = e.common().extended_data.get_record(APP_STRUCT)?;
     if rec.values.len() < 5 {
@@ -101,6 +237,11 @@ fn read_structure(e: &EntityType) -> Option<StructRec> {
         EntityType::Circle(c) => (c.center.x, c.center.y),
         _ => return None,
     };
+    let tc_inlet = if rec.values.len() >= 6 {
+        real(&rec.values[5]).unwrap_or(10.0)
+    } else {
+        10.0
+    };
     Some(StructRec {
         handle: e.common().handle,
         kind,
@@ -108,6 +249,7 @@ fn read_structure(e: &EntityType) -> Option<StructRec> {
         rim: real(&rec.values[2])?,
         area: real(&rec.values[3])?,
         c: real(&rec.values[4])?,
+        tc_inlet,
         x,
         y,
     })
@@ -135,19 +277,233 @@ fn read_pipe(e: &EntityType) -> Option<PipeRec> {
     })
 }
 
-/// Build a `stormsewer::Network` from drawn entities. Structures become nodes
-/// (named N1, N2, … in encounter order); pipes become links, mapped to nodes
-/// by the handles stored in their XDATA.
-pub fn network_from_entities<'a>(entities: impl Iterator<Item = &'a EntityType>) -> Result<Network, String> {
+fn polyline_vertices(pl: &LwPolyline) -> Vec<(f64, f64)> {
+    pl.vertices.iter().map(|v| (v.location.x, v.location.y)).collect()
+}
+
+fn read_catchment(e: &EntityType) -> Option<CatchmentRec> {
+    let EntityType::LwPolyline(pl) = e else {
+        return None;
+    };
+    if !pl.is_closed || pl.vertices.len() < 3 {
+        return None;
+    }
+    let rec = e.common().extended_data.get_record(APP_CATCHMENT)?;
+    if rec.values.len() < 4 {
+        return None;
+    }
+    let verts = polyline_vertices(pl);
+    let area_ac = sqft_to_acres(shoelace_area_sqft(&verts));
+    let inlet = handle(&rec.values[3]).filter(|h| !h.is_null());
+    Some(CatchmentRec {
+        c: real(&rec.values[0])?,
+        flow_length_ft: real(&rec.values[1])?,
+        slope: real(&rec.values[2])?,
+        inlet_handle: inlet,
+        area_ac,
+        centroid: polygon_centroid(&verts),
+    })
+}
+
+/// Replace the diameter on a storm-sewer pipe line entity.
+pub fn set_pipe_diameter(e: &mut EntityType, new_dia: f64) -> bool {
+    let EntityType::Line(_) = e else {
+        return false;
+    };
+    let xd = &mut e.common_mut().extended_data;
+    let Some(old) = xd.records().iter().find(|r| r.application_name == APP_PIPE) else {
+        return false;
+    };
+    if old.values.len() < 4 {
+        return false;
+    };
+    let Some(n) = real(&old.values[1]) else {
+        return false;
+    };
+    let Some(from) = handle(&old.values[2]) else {
+        return false;
+    };
+    let Some(to) = handle(&old.values[3]) else {
+        return false;
+    };
+    let kept: Vec<_> = xd
+        .records()
+        .iter()
+        .filter(|r| r.application_name != APP_PIPE)
+        .cloned()
+        .collect();
+    xd.clear();
+    for r in kept {
+        xd.add_record(r);
+    }
+    xd.add_record(pipe_xdata(new_dia, n, from, to));
+    true
+}
+
+/// Recompute Tc from catchments and write onto structure entities in the document.
+pub fn apply_tc_in_document<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    entities_mut: impl Iterator<Item = &'a mut EntityType>,
+) -> Result<usize, String> {
+    let drawn = drawn_network_from_entities(entities)?;
+    let mut tc_by_handle: HashMap<Handle, f64> = HashMap::new();
+    for (node, &h) in drawn.network.nodes.iter().zip(drawn.node_handles.iter()) {
+        if node.kind != NodeKind::Outfall {
+            tc_by_handle.insert(h, node.tc_inlet);
+        }
+    }
+    let mut updated = 0;
+    for ent in entities_mut {
+        let h = ent.common().handle;
+        let Some(&tc) = tc_by_handle.get(&h) else {
+            continue;
+        };
+        let EntityType::Circle(_) = ent else { continue };
+        let xd = &mut ent.common_mut().extended_data;
+        let Some(old) = xd.records().iter().find(|r| r.application_name == APP_STRUCT).cloned() else {
+            continue;
+        };
+        if old.values.len() < 5 {
+            continue;
+        }
+        let kind = match &old.values[0] {
+            XDataValue::String(s) => parse_kind(s),
+            _ => continue,
+        };
+        let invert = real(&old.values[1]).unwrap_or(0.0);
+        let rim = real(&old.values[2]).unwrap_or(0.0);
+        let area = real(&old.values[3]).unwrap_or(0.0);
+        let c = real(&old.values[4]).unwrap_or(0.0);
+        let kept: Vec<_> = xd
+            .records()
+            .iter()
+            .filter(|r| r.application_name != APP_STRUCT)
+            .cloned()
+            .collect();
+        xd.clear();
+        for r in kept {
+            xd.add_record(r);
+        }
+        xd.add_record(structure_xdata_tc(kind, invert, rim, area, c, tc));
+        updated += 1;
+    }
+    Ok(updated)
+}
+
+/// Write computed inlet Tc back onto structure circle entities (by handle order).
+pub fn apply_tc_to_structures(entities: &mut [EntityType], drawn: &DrawnNetwork) -> usize {
+    let mut updated = 0;
+    for (node, &h) in drawn.network.nodes.iter().zip(drawn.node_handles.iter()) {
+        if node.kind == NodeKind::Outfall {
+            continue;
+        }
+        let Some(ent) = entities.iter_mut().find(|e| e.common().handle == h) else {
+            continue;
+        };
+        let EntityType::Circle(_) = ent else { continue };
+        let xd = &mut ent.common_mut().extended_data;
+        let Some(old) = xd.records().iter().find(|r| r.application_name == APP_STRUCT).cloned() else {
+            continue;
+        };
+        if old.values.len() < 5 {
+            continue;
+        }
+        let kind = match &old.values[0] {
+            XDataValue::String(s) => parse_kind(s),
+            _ => continue,
+        };
+        let invert = real(&old.values[1]).unwrap_or(node.invert);
+        let rim = real(&old.values[2]).unwrap_or(node.rim);
+        let area = real(&old.values[3]).unwrap_or(node.area_ac);
+        let c = real(&old.values[4]).unwrap_or(node.c);
+        let kept: Vec<_> = xd
+            .records()
+            .iter()
+            .filter(|r| r.application_name != APP_STRUCT)
+            .cloned()
+            .collect();
+        xd.clear();
+        for r in kept {
+            xd.add_record(r);
+        }
+        xd.add_record(structure_xdata_tc(kind, invert, rim, area, c, node.tc_inlet));
+        updated += 1;
+    }
+    updated
+}
+
+fn nearest_drainage_structure(structs: &[StructRec], point: (f64, f64)) -> Option<usize> {
+    structs
+        .iter()
+        .enumerate()
+        .filter(|(_, s)| s.kind != NodeKind::Outfall)
+        .min_by(|(_, a), (_, b)| {
+            let da = (a.x - point.0).powi(2) + (a.y - point.1).powi(2);
+            let db = (b.x - point.0).powi(2) + (b.y - point.1).powi(2);
+            da.partial_cmp(&db).unwrap_or(std::cmp::Ordering::Equal)
+        })
+        .map(|(i, _)| i)
+}
+
+fn apply_catchments(structs: &mut [StructRec], catchments: &[CatchmentRec]) {
+    for cat in catchments {
+        let target = if let Some(h) = cat.inlet_handle {
+            structs.iter().position(|s| s.handle == h)
+        } else {
+            nearest_drainage_structure(structs, cat.centroid)
+        };
+        let Some(idx) = target else { continue };
+        let s = &mut structs[idx];
+        let local_ca = s.c * s.area;
+        let add_ca = cat.c * cat.area_ac;
+        let total_area = s.area + cat.area_ac;
+        if total_area > 0.0 {
+            s.area = total_area;
+            s.c = (local_ca + add_ca) / total_area;
+        }
+        let flow_len = if cat.flow_length_ft > 0.0 {
+            cat.flow_length_ft
+        } else {
+            default_flow_length_ft(cat.centroid, (s.x, s.y))
+        };
+        let slope = if cat.slope > 0.0 { cat.slope } else { 0.01 };
+        let tc = catchment_tc_minutes(flow_len, slope);
+        s.tc_inlet = s.tc_inlet.max(tc);
+    }
+}
+
+/// Build a [`DrawnNetwork`] from entities. Structures become nodes (N1, N2, …);
+/// pipes become links mapped by structure handles in XDATA.
+pub fn drawn_network_from_entities<'a>(entities: impl Iterator<Item = &'a EntityType>) -> Result<DrawnNetwork, String> {
     let mut structs: Vec<StructRec> = Vec::new();
-    let mut pipes_raw: Vec<PipeRec> = Vec::new();
+    let mut pipes_raw: Vec<(Handle, PipeRec)> = Vec::new();
+    let mut catchments: Vec<CatchmentRec> = Vec::new();
     for e in entities {
         if let Some(s) = read_structure(e) {
             structs.push(s);
         } else if let Some(p) = read_pipe(e) {
-            pipes_raw.push(p);
+            pipes_raw.push((e.common().handle, p));
+        } else if let Some(c) = read_catchment(e) {
+            catchments.push(c);
         }
     }
+    apply_catchments(&mut structs, &catchments);
+    let network = assemble_network(&structs, &pipes_raw.iter().map(|(_, p)| p.clone()).collect::<Vec<_>>())?;
+    Ok(DrawnNetwork {
+        network,
+        node_handles: structs.iter().map(|s| s.handle).collect(),
+        pipe_handles: pipes_raw.iter().map(|(h, _)| *h).collect(),
+    })
+}
+
+/// Build a `stormsewer::Network` from drawn entities. Structures become nodes
+/// (named N1, N2, … in encounter order); pipes become links, mapped to nodes
+/// by the handles stored in their XDATA.
+pub fn network_from_entities<'a>(entities: impl Iterator<Item = &'a EntityType>) -> Result<Network, String> {
+    Ok(drawn_network_from_entities(entities)?.network)
+}
+
+fn assemble_network(structs: &[StructRec], pipes_raw: &[PipeRec]) -> Result<Network, String> {
     if structs.is_empty() {
         return Err("No storm-sewer structures in the drawing — place Inlet/Junction/Outfall first.".into());
     }
@@ -162,6 +518,7 @@ pub fn network_from_entities<'a>(entities: impl Iterator<Item = &'a EntityType>)
             NodeKind::Junction => Node::junction(&id, s.invert, s.rim, s.area, s.c),
             NodeKind::Outfall => Node::outfall(&id, s.invert, s.rim),
         }
+        .with_tc_inlet(s.tc_inlet)
         .at(s.x, s.y);
         nodes.push(node);
     }
@@ -188,8 +545,9 @@ pub fn network_from_entities<'a>(entities: impl Iterator<Item = &'a EntityType>)
 #[cfg(test)]
 mod tests {
     use super::*;
-    use acadrust::types::Vector3;
-    use acadrust::{Circle, Line};
+    use acadrust::entities::LwVertex;
+    use acadrust::types::{Vector2, Vector3};
+    use acadrust::{Circle, Line, LwPolyline};
 
     fn structure(h: u64, kind: NodeKind, x: f64, invert: f64) -> EntityType {
         let mut e = EntityType::Circle(Circle { center: Vector3::new(x, 0.0, 0.0), radius: 3.0, ..Default::default() });
@@ -203,6 +561,26 @@ mod tests {
         e
     }
 
+    fn catchment_poly(h: u64, c: f64, inlet: u64) -> EntityType {
+        let mut pl = LwPolyline::default();
+        pl.is_closed = true;
+        pl.vertices = vec![
+            LwVertex::new(Vector2::new(40.0, -20.0)),
+            LwVertex::new(Vector2::new(60.0, -20.0)),
+            LwVertex::new(Vector2::new(60.0, 20.0)),
+            LwVertex::new(Vector2::new(40.0, 20.0)),
+        ];
+        let mut e = EntityType::LwPolyline(pl);
+        e.common_mut().handle = Handle::new(h);
+        e.common_mut().extended_data.add_record(catchment_xdata(
+            c,
+            2500.0,
+            0.02,
+            if inlet == 0 { Handle::NULL } else { Handle::new(inlet) },
+        ));
+        e
+    }
+
     #[test]
     fn reconstructs_network_from_tagged_entities() {
         let ents = vec![
@@ -219,12 +597,51 @@ mod tests {
         assert!((net.pipes[0].diameter - 1.5).abs() < 1e-9);
     }
 
+    #[test]
+    fn catchment_adds_area_and_tc_to_inlet() {
+        let ents = vec![
+            structure(1, NodeKind::Inlet, 0.0, 104.0),
+            structure(2, NodeKind::Outfall, 100.0, 100.0),
+            pipe(1, 2, 0.0, 100.0),
+            catchment_poly(10, 0.8, 1),
+        ];
+        let net = network_from_entities(ents.iter()).unwrap();
+        let n1 = &net.nodes[0];
+        assert!(n1.area_ac > 1.0, "catchment should add area, got {}", n1.area_ac);
+        assert!(n1.tc_inlet > 10.0, "Kirpich tc should exceed default 10 min");
+    }
+
     #[test]
     fn errors_when_no_structures() {
         let ents: Vec<EntityType> = vec![];
         assert!(network_from_entities(ents.iter()).is_err());
     }
 
+    #[test]
+    fn set_pipe_diameter_updates_xdata() {
+        let mut e = pipe(1, 2, 0.0, 100.0);
+        assert!(set_pipe_diameter(&mut e, 2.0));
+        let rec = e.common().extended_data.get_record(APP_PIPE).unwrap();
+        assert!((real(&rec.values[0]).unwrap() - 2.0).abs() < 1e-9);
+    }
+
+    #[test]
+    fn nearest_structure_respects_click_tolerance() {
+        let ents = vec![
+            structure(1, NodeKind::Inlet, 0.0, 104.0),
+            structure(2, NodeKind::Outfall, 100.0, 100.0),
+        ];
+        assert_eq!(
+            nearest_structure_at_point(ents.iter(), 1.0, 0.0, 5.0, true),
+            Some(Handle::new(1))
+        );
+        assert!(nearest_drainage_structure_at_point(ents.iter(), 100.0, 0.0, 5.0).is_none());
+        assert_eq!(
+            nearest_drainage_structure_at_point(ents.iter(), 1.0, 0.0, 5.0),
+            Some(Handle::new(1))
+        );
+    }
+
     #[test]
     fn dangling_pipe_is_reported() {
         let ents = vec![
@@ -233,4 +650,4 @@ mod tests {
         ];
         assert!(network_from_entities(ents.iter()).is_err());
     }
-}
+}

src/modules/storm_sewer/dispatch.rs

@@ -0,0 +1,172 @@
+// Storm Sewer command handlers — all domain logic lives here, not in `src/plugin/`.
+
+use crate::command::CadCommand;
+use crate::plugin::host::HostSession;
+
+use super::analysis;
+use super::catchment::{apply_tc_from_network, TagCatchment};
+use super::landxml_import;
+use super::manifest::PLUGIN_ID;
+use super::params_cmd;
+use super::sizing;
+use super::state::StormTabState;
+use super::structures::{PlacePipe, PlaceStructure};
+use super::{data, style};
+
+fn tab_params(host: &mut HostSession<'_>) -> stormsewer::params::StormAnalysisParams {
+    host.ensure_plugin_state(PLUGIN_ID, StormTabState::default)
+        .params()
+        .clone()
+}
+
+/// Handle any `SS_*` command. Returns true when consumed.
+pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+    if !cmd.starts_with("SS_") {
+        return false;
+    }
+
+    match cmd {
+        "SS_ANALYZE" => {
+            let params = tab_params(host);
+            match analysis::analyze_doc(host.entities(), &params) {
+                Ok((ents, report, analysis)) => {
+                    for e in ents {
+                        let _ = host.add_entity(e);
+                    }
+                    if let Ok(drawn) = data::drawn_network_from_entities(host.entities()) {
+                        host.push_undo("SS_STYLE");
+                        let (sur, flood) = style::apply_analysis_style(host.entities_mut(), &drawn, &analysis);
+                        if sur > 0 || flood > 0 {
+                            host.set_dirty();
+                            host.push_info(&format!(
+                                "Styled {sur} surcharged pipe(s), {flood} flooded structure(s)."
+                            ));
+                        }
+                    }
+                    host.bump_geometry();
+                    host.push_info(&format!("Storm sewer analyzed ({}).", params.summary()));
+                    for line in report.lines() {
+                        host.push_output(line);
+                    }
+                }
+                Err(e) => host.push_error(&e),
+            }
+            true
+        }
+        "SS_REPORT" => {
+            let params = tab_params(host);
+            match analysis::report_doc(host.entities(), &params) {
+                Ok(report) => {
+                    for line in report.lines() {
+                        host.push_output(line);
+                    }
+                }
+                Err(e) => host.push_error(&e),
+            }
+            true
+        }
+        "SS_MULTIRP" => {
+            let params = tab_params(host);
+            match analysis::multi_rp_report(host.entities(), &params) {
+                Ok(report) => {
+                    for line in report.lines() {
+                        host.push_output(line);
+                    }
+                }
+                Err(e) => host.push_error(&e),
+            }
+            true
+        }
+        "SS_PROFILE" => {
+            let params = tab_params(host);
+            match analysis::profile_doc(host.entities(), &params) {
+                Ok(ents) => {
+                    for e in ents {
+                        let _ = host.add_entity(e);
+                    }
+                    host.bump_geometry();
+                    host.push_info("Storm sewer HGL profile drawn.");
+                }
+                Err(e) => host.push_error(&e),
+            }
+            true
+        }
+        "SS_SIZE" => {
+            let params = tab_params(host);
+            match sizing::plan_size_updates(host.entities(), &params) {
+                Ok((updates, report, pending)) => {
+                    for line in report.lines() {
+                        host.push_output(line);
+                    }
+                    if pending == 0 {
+                        host.push_info("Storm sewer: all pipes already meet sizing criteria.");
+                    } else {
+                        host.push_undo("SS_SIZE");
+                        let applied = sizing::apply_updates(host.entities_mut(), &updates);
+                        host.bump_geometry();
+                        host.set_dirty();
+                        host.push_info(&format!("Storm sewer: applied {applied} pipe diameter update(s)."));
+                    }
+                }
+                Err(e) => host.push_error(&e),
+            }
+            true
+        }
+        "SS_INLET" | "SS_JUNCTION" | "SS_OUTFALL" => {
+            let c = match cmd {
+                "SS_INLET" => PlaceStructure::inlet(),
+                "SS_JUNCTION" => PlaceStructure::junction(),
+                _ => PlaceStructure::outfall(),
+            };
+            host.push_info(&c.prompt());
+            host.set_active_command(Box::new(c));
+            true
+        }
+        "SS_PIPE" => {
+            let c = PlacePipe::new();
+            host.push_info(&c.prompt());
+            host.set_active_command(Box::new(c));
+            true
+        }
+        "SS_CATCHMENT" => {
+            let c = TagCatchment::new();
+            host.push_info(&c.prompt());
+            host.set_active_command(Box::new(c));
+            true
+        }
+        "SS_APPLYTC" => {
+            host.push_undo("SS_APPLYTC");
+            let snapshot: Vec<_> = host.entities().cloned().collect();
+            match apply_tc_from_network(snapshot.iter(), host.entities_mut()) {
+                Ok(n) => {
+                    host.set_dirty();
+                    host.bump_geometry();
+                    host.push_info(&format!("Storm sewer: updated inlet Tc on {n} structure(s)."));
+                }
+                Err(e) => host.push_error(&e),
+            }
+            true
+        }
+        "SS_LANDXML" | "SS_IMPORTXML" => {
+            match landxml_import::pick_landxml_file() {
+                None => host.push_info("LandXML import cancelled."),
+                Some(Ok(xml)) => match landxml_import::import_landxml(host, &xml) {
+                    Ok(msg) => host.push_info(&msg),
+                    Err(e) => host.push_error(&e),
+                },
+                Some(Err(e)) => host.push_error(&e),
+            }
+            true
+        }
+        cmd if cmd == "SS_PARAMS" || cmd.starts_with("SS_PARAMS ") => {
+            let rest = cmd.trim_start_matches("SS_PARAMS").trim();
+            let state = host.ensure_plugin_state(PLUGIN_ID, StormTabState::default);
+            match params_cmd::apply_params(state, rest) {
+                Ok(msg) => host.push_info(&msg),
+                Err(e) => host.push_error(&e),
+            }
+            true
+        }
+        _ => false,
+    }
+}

src/modules/storm_sewer/headless.rs

@@ -0,0 +1,286 @@
+//! Headless tests for storm-sewer UX: structure picking, C3D-style acquisition,
+//! catchment/pipe command flows (no GUI, no file I/O).
+
+#[cfg(test)]
+mod tests {
+    use acadrust::entities::LwVertex;
+    use acadrust::types::{Vector2, Vector3};
+    use acadrust::xdata::XDataValue;
+    use acadrust::{Circle, EntityType, Handle, Line, LwPolyline};
+    use glam::Vec3;
+
+    use stormsewer::network::NodeKind;
+
+    use crate::command::{CadCommand, CmdResult};
+    use crate::modules::storm_sewer::catchment::TagCatchment;
+    use crate::modules::storm_sewer::data::{
+        self, catchment_xdata, structure_at_point, structure_xdata, APP_CATCHMENT, APP_PIPE,
+    };
+    use crate::modules::storm_sewer::preview;
+    use crate::modules::storm_sewer::structures::PlacePipe;
+    use crate::scene::{Scene, WireModel};
+    use crate::snap::{SnapType, ALL_SNAP_MODES};
+
+    fn structure(h: u64, kind: NodeKind, x: f64, y: f64, radius: f64) -> EntityType {
+        let mut e = EntityType::Circle(Circle {
+            center: Vector3::new(x, y, 0.0),
+            radius,
+            ..Default::default()
+        });
+        e.common_mut().handle = Handle::new(h);
+        e.common_mut()
+            .extended_data
+            .add_record(structure_xdata(kind, 100.0, 106.0, 1.0, 0.7));
+        e
+    }
+
+    fn closed_poly(h: u64) -> EntityType {
+        let mut pl = LwPolyline::default();
+        pl.is_closed = true;
+        pl.vertices = vec![
+            LwVertex::new(Vector2::new(0.0, 0.0)),
+            LwVertex::new(Vector2::new(100.0, 0.0)),
+            LwVertex::new(Vector2::new(100.0, 100.0)),
+            LwVertex::new(Vector2::new(0.0, 100.0)),
+        ];
+        let mut e = EntityType::LwPolyline(pl);
+        e.common_mut().handle = Handle::new(h);
+        e
+    }
+
+    fn advance_catchment_to_inlet(cmd: &mut TagCatchment, poly: EntityType) {
+        cmd.inject_picked_entity(poly);
+        let _ = cmd.on_entity_pick(Handle::new(1), Vec3::ZERO);
+        let _ = cmd.on_enter(); // RunoffC -> FlowLength
+        let _ = cmd.on_enter(); // FlowLength -> Slope
+        let _ = cmd.on_enter(); // Slope -> PickInlet
+    }
+
+    // ── Structure resolution (point pick engine) ────────────────────────────
+
+    #[test]
+    fn structure_at_point_returns_kind_label_and_center() {
+        let ents = vec![structure(1, NodeKind::Junction, 50.0, 25.0, 4.0)];
+        let pick = structure_at_point(ents.iter(), 50.0, 25.0, 5.0, true).unwrap();
+        assert_eq!(pick.handle, Handle::new(1));
+        assert_eq!(pick.label(), "Junction");
+        assert!((pick.x - 50.0).abs() < 1e-9);
+        assert!((pick.y - 25.0).abs() < 1e-9);
+    }
+
+    #[test]
+    fn structure_at_point_prefers_nearest_marker() {
+        let ents = vec![
+            structure(1, NodeKind::Inlet, 0.0, 0.0, 3.0),
+            structure(2, NodeKind::Inlet, 40.0, 0.0, 3.0),
+        ];
+        let pick = structure_at_point(ents.iter(), 38.0, 0.0, 5.0, true).unwrap();
+        assert_eq!(pick.handle, Handle::new(2));
+    }
+
+    #[test]
+    fn catchment_pick_excludes_outfall() {
+        let ents = vec![structure(9, NodeKind::Outfall, 0.0, 0.0, 6.0)];
+        assert!(structure_at_point(ents.iter(), 0.0, 0.0, 20.0, false).is_none());
+        assert!(data::nearest_drainage_structure_at_point(ents.iter(), 0.0, 0.0, 20.0).is_none());
+    }
+
+    #[test]
+    fn pipe_pick_includes_outfall() {
+        let ents = vec![structure(9, NodeKind::Outfall, 0.0, 0.0, 6.0)];
+        let pick = structure_at_point(ents.iter(), 0.0, 0.0, 20.0, true).unwrap();
+        assert_eq!(pick.label(), "Outfall");
+    }
+
+    // ── C3D acquisition constants ───────────────────────────────────────────
+
+    #[test]
+    fn object_pick_not_in_osnap_palette() {
+        assert!(!ALL_SNAP_MODES.iter().any(|(t, _, _)| *t == SnapType::ObjectPick));
+    }
+
+    #[test]
+    fn pick_highlight_color_is_orange() {
+        assert!(WireModel::PICK_HIGHLIGHT[0] > 0.9);
+        assert!(WireModel::PICK_HIGHLIGHT[1] > 0.4);
+        assert!(WireModel::PICK_HIGHLIGHT[2] < 0.2);
+    }
+
+    #[test]
+    fn pipe_rubber_band_connects_start_to_cursor() {
+        let w = preview::pipe_run_rubber_band(10.0, 20.0, Vec3::new(50.0, 60.0, 0.0));
+        assert_eq!(w.points.len(), 2);
+        assert!((w.points[0][0] - 10.0).abs() < 1e-6);
+        assert!((w.points[1][0] - 50.0).abs() < 1e-6);
+        assert_eq!(w.color, WireModel::CYAN);
+    }
+
+    // ── Scene-integrated structure under cursor ─────────────────────────────
+
+    #[test]
+    fn scene_structure_under_cursor_at_marker_center() {
+        let mut scene = Scene::new();
+        let ent = structure(1, NodeKind::Inlet, 200.0, 150.0, 3.0);
+        scene.add_entity(ent);
+        let pick = preview::structure_under_cursor(&scene, 200.0, 150.0, true).unwrap();
+        assert_eq!(pick.label(), "Inlet");
+        let wires = preview::structure_acquire_previews(&scene, Vec3::new(200.0, 150.0, 0.0), true);
+        assert!(!wires.is_empty());
+        assert_eq!(wires[0].color, WireModel::PICK_HIGHLIGHT);
+        assert!(wires[0].line_weight_px >= 3.0);
+    }
+
+    // ── SS_CATCHMENT command flow ───────────────────────────────────────────
+
+    #[test]
+    fn catchment_uses_structure_pick_only_on_inlet_step() {
+        let mut cmd = TagCatchment::new();
+        assert!(cmd.needs_entity_pick());
+        assert!(!cmd.needs_structure_point_pick());
+
+        advance_catchment_to_inlet(&mut cmd, closed_poly(10));
+        assert!(!cmd.needs_entity_pick());
+        assert!(cmd.needs_structure_point_pick());
+        assert!(cmd.prompt().contains("orange snap"));
+    }
+
+    #[test]
+    fn catchment_prompt_includes_acquisition_hint() {
+        let mut cmd = TagCatchment::new();
+        advance_catchment_to_inlet(&mut cmd, closed_poly(10));
+        cmd.set_acquisition_hint(Some("Inlet"));
+        assert!(cmd.prompt().contains("[Inlet]"));
+    }
+
+    #[test]
+    fn catchment_explicit_inlet_writes_xdata_and_ends_command() {
+        let mut cmd = TagCatchment::new();
+        let poly = closed_poly(10);
+        advance_catchment_to_inlet(&mut cmd, poly);
+
+        match cmd.on_structure_pick(Handle::new(42), Vec3::new(10.0, 10.0, 0.0)) {
+            CmdResult::ReplaceMany(replacements, additions) => {
+                assert!(additions.is_empty());
+                assert_eq!(replacements.len(), 1);
+                let (_, ents) = &replacements[0];
+                let ent = &ents[0];
+                let rec = ent.common().extended_data.get_record(APP_CATCHMENT).unwrap();
+                assert!(matches!(&rec.values[3], XDataValue::Handle(h) if *h == Handle::new(42)));
+            }
+            other => panic!("expected ReplaceMany, got {:?}", std::mem::discriminant(&other)),
+        }
+    }
+
+    #[test]
+    fn catchment_enter_at_inlet_auto_assigns_nearest() {
+        let mut cmd = TagCatchment::new();
+        advance_catchment_to_inlet(&mut cmd, closed_poly(10));
+
+        match cmd.on_enter() {
+            CmdResult::ReplaceMany(replacements, _) => {
+                let ent = &replacements[0].1[0];
+                let rec = ent.common().extended_data.get_record(APP_CATCHMENT).unwrap();
+                assert!(matches!(&rec.values[3], XDataValue::Handle(h) if h.is_null()));
+            }
+            other => panic!("expected ReplaceMany, got {:?}", std::mem::discriminant(&other)),
+        }
+    }
+
+    #[test]
+    fn catchment_structure_pick_sets_flow_length_when_zero() {
+        let mut cmd = TagCatchment::new();
+        advance_catchment_to_inlet(&mut cmd, closed_poly(10));
+        match cmd.on_structure_pick(Handle::new(1), Vec3::new(0.0, 0.0, 0.0)) {
+            CmdResult::ReplaceMany(replacements, _) => {
+                let rec = replacements[0].1[0]
+                    .common()
+                    .extended_data
+                    .get_record(APP_CATCHMENT)
+                    .unwrap();
+                let flow = match &rec.values[1] {
+                    XDataValue::Real(v) => *v,
+                    _ => 0.0,
+                };
+                // Centroid (50,50) → inlet at (0,0) ≈ 70.7 ft
+                assert!(flow > 50.0, "expected auto flow length, got {flow}");
+            }
+            other => panic!("expected ReplaceMany, got {:?}", std::mem::discriminant(&other)),
+        }
+    }
+
+    // ── SS_PIPE command flow ────────────────────────────────────────────────
+
+    #[test]
+    fn pipe_uses_structure_point_pick() {
+        let cmd = PlacePipe::new();
+        assert!(!cmd.needs_entity_pick());
+        assert!(cmd.needs_structure_point_pick());
+        assert!(cmd.prompt().contains("orange snap"));
+    }
+
+    #[test]
+    fn pipe_end_prompt_references_start_structure_label() {
+        let mut cmd = PlacePipe::new();
+        cmd.set_acquisition_hint(Some("Inlet"));
+        cmd.on_structure_pick(Handle::new(1), Vec3::new(0.0, 0.0, 0.0));
+        assert!(cmd.prompt().contains("from Inlet"));
+    }
+
+    #[test]
+    fn pipe_commit_links_structures_in_xdata() {
+        let mut cmd = PlacePipe::new();
+        cmd.on_structure_pick(Handle::new(1), Vec3::new(0.0, 0.0, 0.0));
+        cmd.on_structure_pick(Handle::new(2), Vec3::new(100.0, 0.0, 0.0));
+        let mut cmd = PlacePipe::new();
+        cmd.on_structure_pick(Handle::new(1), Vec3::new(0.0, 0.0, 0.0));
+        if let CmdResult::CommitAndExit(e) = cmd.on_structure_pick(Handle::new(2), Vec3::new(100.0, 0.0, 0.0))
+        {
+            let rec = e.common().extended_data.get_record(APP_PIPE).unwrap();
+            assert!(matches!(&rec.values[2], XDataValue::Handle(h) if *h == Handle::new(1)));
+            assert!(matches!(&rec.values[3], XDataValue::Handle(h) if *h == Handle::new(2)));
+        } else {
+            panic!("second pick should commit pipe");
+        }
+    }
+
+    #[test]
+    fn pipe_preview_only_on_second_pick() {
+        let mut cmd = PlacePipe::new();
+        assert!(cmd.on_preview_wires(Vec3::new(10.0, 0.0, 0.0)).is_empty());
+        cmd.on_structure_pick(Handle::new(1), Vec3::ZERO);
+        assert_eq!(cmd.on_preview_wires(Vec3::new(10.0, 0.0, 0.0)).len(), 1);
+    }
+
+    // ── End-to-end: catchment + network assembly ────────────────────────────
+
+    #[test]
+    fn explicit_catchment_inlet_feeds_network_analysis() {
+        let mut ents = vec![
+            structure(1, NodeKind::Inlet, 0.0, 0.0, 3.0),
+            structure(2, NodeKind::Outfall, 100.0, 0.0, 6.0),
+            EntityType::Line(Line::from_points(
+                Vector3::new(0.0, 0.0, 0.0),
+                Vector3::new(100.0, 0.0, 0.0),
+            )),
+        ];
+        ents[2].common_mut().handle = Handle::new(3);
+        ents[2]
+            .common_mut()
+            .extended_data
+            .add_record(data::pipe_xdata(1.5, 0.013, Handle::new(1), Handle::new(2)));
+
+        let mut poly = closed_poly(10);
+        poly.common_mut()
+            .extended_data
+            .add_record(catchment_xdata(0.8, 2500.0, 0.02, Handle::new(1)));
+        ents.push(poly);
+
+        let net = data::network_from_entities(ents.iter()).unwrap();
+        assert!(net.nodes[0].area_ac > 1.0, "catchment area should merge onto inlet");
+        assert!(
+            net.nodes[0].tc_inlet > 10.0,
+            "Kirpich tc should exceed default, got {}",
+            net.nodes[0].tc_inlet
+        );
+    }
+}

src/modules/storm_sewer/landxml_import.rs

@@ -0,0 +1,118 @@
+// LandXML pipe-network import → storm-sewer drawing entities.
+
+use std::collections::HashMap;
+
+use acadrust::types::Vector3;
+use acadrust::{Circle, EntityType, Handle, Line};
+
+use stormsewer::io::landxml::{parse_landxml, LandXmlStruct};
+
+use super::data;
+use crate::plugin::host::HostSession;
+
+fn structure_entity(s: &LandXmlStruct, radius: f64) -> EntityType {
+    let mut e = EntityType::Circle(Circle {
+        center: Vector3::new(s.x, s.y, 0.0),
+        radius,
+        ..Default::default()
+    });
+    let (area, c) = if s.kind == stormsewer::network::NodeKind::Outfall {
+        (0.0, 0.0)
+    } else {
+        (s.area_ac, s.c)
+    };
+    e.common_mut()
+        .extended_data
+        .add_record(data::structure_xdata(s.kind, s.invert, s.rim, area, c));
+    e
+}
+
+fn pipe_entity(diameter: f64, n: f64, from: Handle, to: Handle, x0: f64, y0: f64, x1: f64, y1: f64) -> EntityType {
+    let mut e = EntityType::Line(Line::from_points(
+        Vector3::new(x0, y0, 0.0),
+        Vector3::new(x1, y1, 0.0),
+    ));
+    e.common_mut()
+        .extended_data
+        .add_record(data::pipe_xdata(diameter, n, from, to));
+    e
+}
+
+/// Open a LandXML file dialog and return the file text.
+pub fn pick_landxml_file() -> Option<Result<String, String>> {
+    let path = rfd::FileDialog::new()
+        .add_filter("LandXML", &["xml", "landxml"])
+        .set_title("Import LandXML pipe network")
+        .pick_file()?;
+    Some(std::fs::read_to_string(&path).map_err(|e| format!("cannot read {}: {e}", path.display())))
+}
+
+/// Import a LandXML document into the active drawing tab.
+pub fn import_landxml(host: &mut HostSession<'_>, xml: &str) -> Result<String, String> {
+    let doc = parse_landxml(xml)?;
+    let net = doc.primary_network()?.clone();
+
+    if net.structures.is_empty() {
+        return Err("LandXML: no structures to import".into());
+    }
+
+    host.push_undo("SS_LANDXML");
+
+    let mut name_to_handle: HashMap<String, Handle> = HashMap::new();
+    let mut coord: HashMap<String, (f64, f64)> = HashMap::new();
+
+    for s in &net.structures {
+        let radius = match s.kind {
+            stormsewer::network::NodeKind::Outfall => 6.0,
+            stormsewer::network::NodeKind::Inlet => 3.0,
+            stormsewer::network::NodeKind::Junction => 4.0,
+        };
+        let ent = structure_entity(s, radius);
+        let h = host.add_entity(ent);
+        name_to_handle.insert(s.name.clone(), h);
+        coord.insert(s.name.clone(), (s.x, s.y));
+    }
+
+    let mut pipe_count = 0;
+    for p in &net.pipes {
+        let Some(&from_h) = name_to_handle.get(&p.from) else {
+            continue;
+        };
+        let Some(&to_h) = name_to_handle.get(&p.to) else {
+            continue;
+        };
+        let (x0, y0) = coord.get(&p.from).copied().unwrap_or((0.0, 0.0));
+        let (x1, y1) = coord.get(&p.to).copied().unwrap_or((0.0, 0.0));
+        host.add_entity(pipe_entity(p.diameter_ft, p.n, from_h, to_h, x0, y0, x1, y1));
+        pipe_count += 1;
+    }
+
+    if pipe_count == 0 {
+        return Err("LandXML: structures imported but no pipes could be connected — check StartStruct/EndStruct names.".into());
+    }
+
+    host.bump_geometry();
+    host.set_dirty();
+
+    Ok(format!(
+        "Imported LandXML \"{}\": {} structure(s), {} pipe(s).",
+        net.name,
+        net.structures.len(),
+        pipe_count
+    ))
+}
+
+#[cfg(test)]
+mod tests {
+    use stormsewer::io::landxml::parse_landxml;
+
+    const SAMPLE: &str = include_str!("../../../crates/stormsewer/examples/sample_landxml.xml");
+
+    #[test]
+    fn sample_xml_parses_three_structures() {
+        let doc = parse_landxml(SAMPLE).unwrap();
+        let net = doc.primary_network().unwrap();
+        assert_eq!(net.structures.len(), 3);
+        assert_eq!(net.pipes.len(), 2);
+    }
+}

src/modules/storm_sewer/manifest.rs

@@ -0,0 +1,16 @@
+// Storm Sewer plugin identity (domain-specific; not part of `src/plugin/` runtime).
+
+use crate::plugin::manifest::{ApiVersion, PluginManifest};
+
+pub const PLUGIN_ID: &str = "opencad.storm_sewer";
+
+pub static MANIFEST: PluginManifest = PluginManifest {
+    id: PLUGIN_ID,
+    name: "Storm Sewer",
+    version: "0.2.0",
+    description: "Gravity storm-drain network design and analysis",
+    api_version: ApiVersion::CURRENT,
+    ribbon_order: 50,
+    xdata_apps: &["STORMSEWER_STRUCT", "STORMSEWER_PIPE", "STORMSEWER_CATCHMENT"],
+    command_prefixes: &["SS_"],
+};

src/modules/storm_sewer/mod.rs

@@ -1,14 +1,24 @@
-// Storm Sewer module — gravity storm-drain network design & analysis.
+// Storm Sewer add-on (`opencad.storm_sewer`) — gravity storm-drain design & analysis.
 //
-// Implements the standard public-domain methods (Rational method, Manning,
-// HGL backwater) via the external `stormsewer` engine crate. The ribbon tab
-// here is the UI surface; the actual command handlers (place structure, draw
-// pipe, run analysis) are dispatched by the host command system — see
-// INTEGRATION.md for where each `SS_*` command plugs in.
+// Package layout follows `docs/plugin-architecture.md` (QGIS-style add-on).
+// Ribbon: `CadModule` here; commands: `dispatch.rs` via `BuiltinPlugin`; engine: `stormsewer` crate.
 
 pub mod analysis;
+pub mod catchment;
 pub mod data;
+pub mod dispatch;
+#[cfg(test)]
+mod headless;
+pub mod landxml_import;
+pub mod manifest;
+pub mod params_cmd;
+pub mod preview;
+pub mod plugin;
+pub mod register;
+pub mod sizing;
+pub mod state;
 pub mod structures;
+pub mod style;
 
 use crate::modules::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
 
@@ -21,7 +31,14 @@ inventory::submit!(crate::command::CommandRegistration {
         "SS_JUNCTION",
         "SS_OUTFALL",
         "SS_PIPE",
+        "SS_CATCHMENT",
+        "SS_APPLYTC",
+        "SS_LANDXML",
+        "SS_IMPORTXML",
         "SS_ANALYZE",
+        "SS_SIZE",
+        "SS_PARAMS",
+        "SS_MULTIRP",
         "SS_REPORT",
         "SS_PROFILE",
     ]
@@ -32,6 +49,7 @@ const IC_JUNCTION: &[u8] = include_bytes!("icons/junction.svg");
 const IC_OUTFALL: &[u8] = include_bytes!("icons/outfall.svg");
 const IC_PIPE: &[u8] = include_bytes!("icons/pipe.svg");
 const IC_ANALYZE: &[u8] = include_bytes!("icons/analyze.svg");
+const IC_SIZE: &[u8] = include_bytes!("icons/pipe.svg");
 const IC_REPORT: &[u8] = include_bytes!("icons/report.svg");
 const IC_PROFILE: &[u8] = include_bytes!("icons/profile.svg");
 
@@ -63,6 +81,8 @@ impl CadModule for StormSewerModule {
                     RibbonItem::LargeTool(tool("SS_JUNCTION", "Junction", IC_JUNCTION)),
                     RibbonItem::LargeTool(tool("SS_OUTFALL", "Outfall", IC_OUTFALL)),
                     RibbonItem::LargeTool(tool("SS_PIPE", "Pipe\nRun", IC_PIPE)),
+                    RibbonItem::Tool(tool("SS_CATCHMENT", "Catchment", IC_INLET)),
+                    RibbonItem::Tool(tool("SS_LANDXML", "Import\nLandXML", IC_PIPE)),
                 ],
             },
             // ── Analysis: run the engine and review results ─────────────────
@@ -70,6 +90,10 @@ impl CadModule for StormSewerModule {
                 title: "Analysis",
                 tools: vec![
                     RibbonItem::LargeTool(tool("SS_ANALYZE", "Analyze", IC_ANALYZE)),
+                    RibbonItem::LargeTool(tool("SS_SIZE", "Size\nPipes", IC_SIZE)),
+                    RibbonItem::Tool(tool("SS_PARAMS", "Params", IC_ANALYZE)),
+                    RibbonItem::Tool(tool("SS_APPLYTC", "Apply Tc", IC_ANALYZE)),
+                    RibbonItem::Tool(tool("SS_MULTIRP", "Multi-RP", IC_REPORT)),
                     RibbonItem::Tool(tool("SS_REPORT", "Report", IC_REPORT)),
                     RibbonItem::Tool(tool("SS_PROFILE", "Profile", IC_PROFILE)),
                 ],
@@ -81,11 +105,11 @@ impl CadModule for StormSewerModule {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::modules::registry;
+    use crate::plugin::all_ribbon_modules;
 
     #[test]
     fn module_is_registered_in_ribbon() {
-        let titles: Vec<&str> = registry::all_modules().iter().map(|m| m.title()).collect();
+        let titles: Vec<&str> = all_ribbon_modules().iter().map(|m| m.title()).collect();
         assert!(titles.contains(&"Storm Sewer"), "ribbon tabs: {titles:?}");
     }
 
@@ -99,7 +123,9 @@ mod tests {
                 }
             }
         }
-        for needed in ["SS_INLET", "SS_PIPE", "SS_ANALYZE", "SS_REPORT", "SS_PROFILE"] {
+        for needed in [
+            "SS_INLET", "SS_PIPE", "SS_ANALYZE", "SS_SIZE", "SS_PARAMS", "SS_MULTIRP", "SS_REPORT", "SS_PROFILE",
+        ] {
             assert!(ids.contains(&needed), "missing {needed}; have {ids:?}");
         }
     }
@@ -187,7 +213,8 @@ PIPE P2 N2 OUT 300 1.5 0.013
             pipe,
         ];
 
-        let (annotations, report) = super::analysis::analyze_doc(ents.iter()).expect("analyze drawn net");
+        let p = super::analysis::default_params();
+        let (annotations, report, _) = super::analysis::analyze_doc(ents.iter(), &p).expect("analyze drawn net");
         assert!(!annotations.is_empty(), "expected flow/HGL labels");
         assert!(report.contains("STORM SEWER ANALYSIS"), "report:\n{report}");
     }

src/modules/storm_sewer/params_cmd.rs

@@ -0,0 +1,137 @@
+// Parse `SS_PARAMS` subcommands and update StormTabState.
+
+use stormsewer::idf::IdfCurve;
+use stormsewer::params::StormAnalysisParams;
+
+use super::state::StormTabState;
+
+fn parse_f64(s: &str) -> Result<f64, String> {
+    s.trim()
+        .replace(',', ".")
+        .parse::<f64>()
+        .map_err(|_| format!("`{s}` is not a number"))
+}
+
+/// Apply `SS_PARAMS …` tokens. Empty rest → show summary.
+pub fn apply_params(state: &mut StormTabState, rest: &str) -> Result<String, String> {
+    let t: Vec<&str> = rest.split_whitespace().collect();
+    if t.is_empty() {
+        return Ok(format!("Storm params: {}", state.params.summary()));
+    }
+    let key = t[0].to_ascii_uppercase();
+    match key.as_str() {
+        "RP" | "RETURN" => {
+            let rp: u32 = t
+                .get(1)
+                .ok_or("SS_PARAMS RP needs return period years (e.g. SS_PARAMS RP 25)")?
+                .parse()
+                .map_err(|_| "return period must be an integer year")?;
+            state.params.idf.set_design_rp(rp);
+            if state.params.idf.curve(rp).is_none() {
+                // Seed a scaled curve from the design curve if missing.
+                let base = state.params.idf.design_curve();
+                let scale = (rp as f64 / 10.0).sqrt().max(1.0);
+                state.params.idf.set_curve(rp, IdfCurve::new(base.a * scale, base.b, base.c));
+            }
+            Ok(format!("Design return period set to {rp} yr."))
+        }
+        "IDF" => {
+            let (rp, ai) = if t.len() == 5 {
+                let rp: u32 = t[1].parse().map_err(|_| "IDF return period must be integer")?;
+                (rp, 2)
+            } else if t.len() == 4 {
+                (state.params.idf.design_rp, 1)
+            } else {
+                return Err("SS_PARAMS IDF [rp] <a> <b> <c>  (e.g. SS_PARAMS IDF 60 10 0.8)".into());
+            };
+            let a = parse_f64(t[ai])?;
+            let b = parse_f64(t[ai + 1])?;
+            let c = parse_f64(t[ai + 2])?;
+            state.params.idf.set_curve(rp, IdfCurve::new(a, b, c));
+            state.params.idf.set_design_rp(rp);
+            Ok(format!("IDF for {rp}-yr set: i = {a}/(t+{b})^{c}"))
+        }
+        "TAILWATER" | "TW" => {
+            let v = t.get(1).map(|s| s.to_ascii_uppercase());
+            match v.as_deref() {
+                None => Err("SS_PARAMS TAILWATER <elev_ft> | NONE".into()),
+                Some("NONE" | "FREE") => {
+                    state.params.hydraulics.tailwater = None;
+                    Ok("Tailwater: free outfall.".into())
+                }
+                Some(s) => {
+                    let elev = parse_f64(s)?;
+                    state.params.hydraulics.tailwater = Some(elev);
+                    Ok(format!("Tailwater elevation set to {elev:.2} ft."))
+                }
+            }
+        }
+        "MINTC" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS MINTC <minutes>")?)?;
+            state.params.hydraulics.min_tc = v;
+            Ok(format!("Minimum Tc set to {v:.1} min."))
+        }
+        "JUNCTIONK" | "JK" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS JUNCTIONK <k>")?)?;
+            state.params.hydraulics.junction_k = v;
+            Ok(format!("Junction loss K set to {v:.2}."))
+        }
+        "VMIN" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS VMIN <ft/s>")?)?;
+            state.params.sizing.min_velocity = v;
+            Ok(format!("Minimum velocity set to {v:.2} ft/s."))
+        }
+        "VMAX" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS VMAX <ft/s>")?)?;
+            state.params.sizing.max_velocity = v;
+            Ok(format!("Maximum velocity set to {v:.2} ft/s."))
+        }
+        "MAXFULL" | "PFULL" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS MAXFULL <percent>")?)?;
+            state.params.sizing.max_pct_full = (v / 100.0).clamp(0.1, 1.0);
+            Ok(format!("Max % full set to {v:.0}%."))
+        }
+        "INLETLEN" | "GRATE" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS INLETLEN <ft>")?)?;
+            state.params.inlet_grate_length_ft = v;
+            Ok(format!("Inlet grate length set to {v:.2} ft."))
+        }
+        "INLETD" | "CURBDEPTH" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS INLETD <ft>")?)?;
+            state.params.inlet_flow_depth_ft = v;
+            Ok(format!("Inlet curb flow depth set to {v:.3} ft."))
+        }
+        "INLETS" | "GUTTERS" => {
+            let v = parse_f64(t.get(1).ok_or("SS_PARAMS INLETS <ft/ft>")?)?;
+            state.params.inlet_gutter_slope = v;
+            Ok(format!("Inlet gutter slope set to {v:.4} ft/ft."))
+        }
+        "RESET" => {
+            state.params = StormAnalysisParams::municipal();
+            Ok("Storm params reset to municipal defaults.".into())
+        }
+        _ => Err(format!(
+            "Unknown SS_PARAMS key `{key}`. Keys: RP, IDF, TAILWATER, MINTC, JUNCTIONK, VMIN, VMAX, MAXFULL, INLETLEN, INLETD, INLETS, RESET"
+        )),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn sets_return_period() {
+        let mut s = StormTabState::default();
+        apply_params(&mut s, "RP 25").unwrap();
+        assert_eq!(s.params.idf.design_rp, 25);
+    }
+
+    #[test]
+    fn sets_idf_coefficients() {
+        let mut s = StormTabState::default();
+        apply_params(&mut s, "IDF 70 12 0.75").unwrap();
+        let c = s.params.idf.design_curve();
+        assert!((c.a - 70.0).abs() < 1e-9);
+    }
+}

src/modules/storm_sewer/plugin.rs

@@ -0,0 +1,24 @@
+// Thin adapter: implements the generic `BuiltinPlugin` trait for Storm Sewer.
+// Domain logic is in `dispatch.rs`; identity in `manifest.rs`; hook in `register.rs`.
+
+use crate::plugin::host::{BuiltinPlugin, HostSession};
+use crate::plugin::manifest::PluginManifest;
+
+use super::dispatch;
+use super::manifest;
+
+pub struct StormSewerPlugin;
+
+impl BuiltinPlugin for StormSewerPlugin {
+    fn manifest(&self) -> &'static PluginManifest {
+        &manifest::MANIFEST
+    }
+
+    fn ribbon(&self) -> Box<dyn crate::modules::CadModule> {
+        Box::new(super::StormSewerModule)
+    }
+
+    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool {
+        dispatch::handle(host, cmd)
+    }
+}

src/modules/storm_sewer/plugin.toml

@@ -0,0 +1,16 @@
+# Storm Sewer add-on metadata.
+# Must stay in sync with manifest.rs (compile-time source of truth for the host).
+
+[plugin]
+id = "opencad.storm_sewer"
+name = "Storm Sewer"
+version = "0.2.0"
+description = "Gravity storm-drain network design and analysis"
+author = "Open CAD Studio contributors"
+license = "GPL-3.0-only"
+
+[opencad]
+api_version = 1
+ribbon_order = 50
+command_prefixes = ["SS_"]
+xdata_apps = ["STORMSEWER_STRUCT", "STORMSEWER_PIPE", "STORMSEWER_CATCHMENT"]

src/modules/storm_sewer/preview.rs

@@ -0,0 +1,96 @@
+// C3D-style acquisition previews: orange structure highlight, pipe rubber-band.
+
+use acadrust::{EntityType, Handle};
+use glam::Vec3;
+
+use crate::scene::{Scene, WireModel};
+
+use super::data::{self, StructurePick};
+
+pub const STRUCTURE_PICK_PAD_FT: f64 = 15.0;
+
+/// Orange highlight wires for a structure under the cursor.
+pub fn highlight_structure(scene: &Scene, pick: &StructurePick) -> Vec<WireModel> {
+    highlight_handles(scene, &[pick.handle], WireModel::PICK_HIGHLIGHT, 3.0)
+}
+
+/// Highlight any entity (e.g. catchment polyline) in acquisition color.
+pub fn highlight_entity(scene: &Scene, handle: Handle) -> Vec<WireModel> {
+    highlight_handles(scene, &[handle], WireModel::PICK_HIGHLIGHT, 2.5)
+}
+
+/// Dim orange fill on closed catchment while hovering.
+pub fn highlight_catchment_poly(scene: &Scene, handle: Handle) -> Vec<WireModel> {
+    let mut wires = highlight_handles(scene, &[handle], WireModel::PICK_HIGHLIGHT_DIM, 2.0);
+    for w in &mut wires {
+        w.line_weight_px = 2.0;
+    }
+    wires
+}
+
+pub fn highlight_handles(
+    scene: &Scene,
+    handles: &[Handle],
+    color: [f32; 4],
+    line_weight_px: f32,
+) -> Vec<WireModel> {
+    let mut out = scene.wire_models_for(handles);
+    for w in &mut out {
+        w.color = color;
+        w.line_weight_px = line_weight_px;
+    }
+    out
+}
+
+/// Resolve the structure under the cursor for storm network commands.
+pub fn structure_under_cursor(
+    scene: &Scene,
+    x: f64,
+    y: f64,
+    catchment_inlet_only: bool,
+) -> Option<StructurePick> {
+    data::structure_at_point(
+        scene.document.entities(),
+        x,
+        y,
+        STRUCTURE_PICK_PAD_FT,
+        !catchment_inlet_only,
+    )
+}
+
+/// Cyan rubber-band from a fixed structure center to the cursor (pipe run preview).
+pub fn pipe_run_rubber_band(from_x: f64, from_y: f64, to: Vec3) -> WireModel {
+    WireModel::solid(
+        "__ss_pipe_preview__".into(),
+        vec![
+            [from_x as f32, from_y as f32, 0.0],
+            [to.x, to.y, to.z],
+        ],
+        WireModel::CYAN,
+        false,
+    )
+}
+
+/// Extra preview wires for storm-sewer structure acquisition at `cursor`.
+pub fn structure_acquire_previews(
+    scene: &Scene,
+    cursor: Vec3,
+    catchment_inlet_only: bool,
+) -> Vec<WireModel> {
+    let Some(pick) = structure_under_cursor(scene, cursor.x as f64, cursor.y as f64, catchment_inlet_only)
+    else {
+        return vec![];
+    };
+    highlight_structure(scene, &pick)
+}
+
+/// Closed polyline under cursor (catchment step 1).
+pub fn catchment_poly_under_cursor(scene: &Scene, handle: Handle) -> Vec<WireModel> {
+    let Some(ent) = scene.document.get_entity(handle) else {
+        return vec![];
+    };
+    if !matches!(ent, EntityType::LwPolyline(pl) if pl.is_closed) {
+        return vec![];
+    }
+    highlight_catchment_poly(scene, handle)
+}

src/modules/storm_sewer/register.rs

@@ -0,0 +1,10 @@
+// Compile-time registration with the generic plugin host.
+// Keep this file free of storm-sewer logic — only the hook.
+
+use super::plugin::StormSewerPlugin;
+
+inventory::submit! {
+    crate::plugin::registry::PluginRegistration {
+        construct: || Box::new(StormSewerPlugin),
+    }
+}

src/modules/storm_sewer/sizing.rs

@@ -0,0 +1,132 @@
+// Apply storm-sewer pipe sizing to the drawn network in the active document.
+
+use acadrust::{EntityType, Handle};
+
+use stormsewer::design::SizeOutcome;
+use stormsewer::params::StormAnalysisParams;
+use stormsewer::report::format_sizing;
+
+use super::data;
+
+/// A pipe diameter change to apply in the document.
+#[derive(Clone, Debug, PartialEq)]
+pub struct PipeDiameterUpdate {
+    pub handle: Handle,
+    pub new_diameter_ft: f64,
+}
+
+/// Size pipes from drawn entities without modifying the document.
+pub fn size_doc_report<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    params: &StormAnalysisParams,
+) -> Result<String, String> {
+    let (_, report, _) = plan_size_updates(entities, params)?;
+    Ok(report)
+}
+
+/// Compute sizing recommendations and the list of pipe updates to apply.
+pub fn plan_size_updates<'a>(
+    entities: impl Iterator<Item = &'a EntityType>,
+    params: &StormAnalysisParams,
+) -> Result<(Vec<PipeDiameterUpdate>, String, usize), String> {
+    let drawn = data::drawn_network_from_entities(entities)?;
+    let (_, recs) = drawn
+        .network
+        .analyze_and_size_params(params)
+        .map_err(|e| e.to_string())?;
+
+    let mut updates = Vec::new();
+    for (handle, rec) in drawn.pipe_handles.iter().zip(recs.iter()) {
+        if rec.outcome == SizeOutcome::NoSolution {
+            continue;
+        }
+        if (rec.recommended_diameter_ft - rec.current_diameter_ft).abs() < 1e-6 {
+            continue;
+        }
+        updates.push(PipeDiameterUpdate {
+            handle: *handle,
+            new_diameter_ft: rec.recommended_diameter_ft,
+        });
+    }
+
+    let report = format_sizing(&recs);
+    let pending = updates.len();
+    Ok((updates, report, pending))
+}
+
+/// Write planned diameter updates onto matching pipe entities.
+pub fn apply_updates<'a>(entities: impl Iterator<Item = &'a mut EntityType>, updates: &[PipeDiameterUpdate]) -> usize {
+    let mut applied = 0usize;
+    for e in entities {
+        let h = e.common().handle;
+        if let Some(u) = updates.iter().find(|u| u.handle == h) {
+            if data::set_pipe_diameter(e, u.new_diameter_ft) {
+                applied += 1;
+            }
+        }
+    }
+    applied
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use acadrust::types::Vector3;
+    use acadrust::{Circle, Line};
+    use stormsewer::network::NodeKind;
+
+    fn mk_struct(h: u64, kind: NodeKind, x: f64, invert: f64) -> EntityType {
+        let mut e = EntityType::Circle(Circle {
+            center: Vector3::new(x, 0.0, 0.0),
+            radius: 3.0,
+            ..Default::default()
+        });
+        e.common_mut().handle = Handle::new(h);
+        e.common_mut()
+            .extended_data
+            .add_record(data::structure_xdata(kind, invert, invert + 6.0, 2.0, 0.7));
+        e
+    }
+
+    fn mk_pipe(from: u64, to: u64, x1: f64, x2: f64, dia: f64) -> EntityType {
+        let mut e = EntityType::Line(Line::from_points(
+            Vector3::new(x1, 0.0, 0.0),
+            Vector3::new(x2, 0.0, 0.0),
+        ));
+        e.common_mut().handle = Handle::new(from + 100);
+        e.common_mut()
+            .extended_data
+            .add_record(data::pipe_xdata(dia, 0.013, Handle::new(from), Handle::new(to)));
+        e
+    }
+
+    #[test]
+    fn plan_size_finds_undersized_trunk() {
+        let ents = vec![
+            mk_struct(1, NodeKind::Inlet, 0.0, 100.0),
+            mk_struct(2, NodeKind::Inlet, 100.0, 99.0),
+            mk_struct(3, NodeKind::Outfall, 200.0, 98.0),
+            mk_pipe(1, 2, 0.0, 100.0, 1.5),
+            mk_pipe(2, 3, 100.0, 200.0, 1.5),
+        ];
+        let p = stormsewer::params::StormAnalysisParams::municipal();
+        let (updates, report, pending) = plan_size_updates(ents.iter(), &p).expect("size");
+        assert!(pending >= 1, "report:\n{report}");
+        assert!(!updates.is_empty());
+    }
+
+    #[test]
+    fn apply_updates_writes_xdata() {
+        let mut ents = vec![
+            mk_struct(1, NodeKind::Inlet, 0.0, 100.0),
+            mk_struct(2, NodeKind::Outfall, 200.0, 98.0),
+            mk_pipe(1, 2, 0.0, 200.0, 0.5),
+        ];
+        let p = stormsewer::params::StormAnalysisParams::municipal();
+        let (updates, _, _) = plan_size_updates(ents.iter(), &p).expect("plan");
+        let applied = apply_updates(ents.iter_mut(), &updates);
+        assert!(applied >= 1);
+        let drawn = data::drawn_network_from_entities(ents.iter()).unwrap();
+        assert!(drawn.network.pipes[0].diameter > 0.5);
+    }
+}

src/modules/storm_sewer/state.rs

@@ -0,0 +1,26 @@
+// Per-document storm-sewer parameters (stored in host tab plugin_state).
+
+use stormsewer::params::StormAnalysisParams;
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct StormTabState {
+    pub params: StormAnalysisParams,
+}
+
+impl Default for StormTabState {
+    fn default() -> Self {
+        Self {
+            params: StormAnalysisParams::municipal(),
+        }
+    }
+}
+
+impl StormTabState {
+    pub fn params(&self) -> &StormAnalysisParams {
+        &self.params
+    }
+
+    pub fn params_mut(&mut self) -> &mut StormAnalysisParams {
+        &mut self.params
+    }
+}

src/modules/storm_sewer/structures.rs

@@ -15,7 +15,9 @@ use glam::Vec3;
 use stormsewer::network::NodeKind;
 
 use super::data;
-use crate::command::{CadCommand, CmdResult};
+use super::preview;
+use crate::command::{CadCommand, CmdResult, ObjectPickHit};
+use crate::scene::{Scene, WireModel};
 
 fn parse_num(text: &str) -> Option<f64> {
     text.trim().replace(',', ".").parse::<f64>().ok()
@@ -113,8 +115,6 @@ impl CadCommand for PlaceStructure {
         None
     }
     fn on_point(&mut self, pt: Vec3) -> CmdResult {
-        // A click always places the structure, using whatever values have been
-        // entered (remaining fields keep their defaults).
         self.commit(pt.x as f64, pt.y as f64)
     }
     fn on_enter(&mut self) -> CmdResult {
@@ -135,11 +135,21 @@ pub struct PlacePipe {
     n: f64,
     start_handle: Option<Handle>,
     start_xy: (f64, f64),
+    start_label: Option<String>,
+    acquire_hint: Option<String>,
 }
 
 impl PlacePipe {
     pub fn new() -> Self {
-        Self { step: PStep::PickStart, diameter: 1.25, n: 0.013, start_handle: None, start_xy: (0.0, 0.0) }
+        Self {
+            step: PStep::PickStart,
+            diameter: 1.25,
+            n: 0.013,
+            start_handle: None,
+            start_xy: (0.0, 0.0),
+            start_label: None,
+            acquire_hint: None,
+        }
     }
     fn commit(&self, end_handle: Handle, ex: f64, ey: f64) -> CmdResult {
         let line = Line::from_points(
@@ -164,25 +174,69 @@ impl CadCommand for PlacePipe {
         "SS_PIPE"
     }
     fn prompt(&self) -> String {
+        let hint = self
+            .acquire_hint
+            .as_deref()
+            .map(|h| format!(" [{h}]"))
+            .unwrap_or_default();
         match self.step {
-            PStep::PickStart => "Pipe: click the START structure:".into(),
-            PStep::PickEnd => format!("Pipe: click the END structure (dia {:.2} ft, n {:.3}):", self.diameter, self.n),
+            PStep::PickStart => format!("Pipe run: click START structure (orange snap){hint}:"),
+            PStep::PickEnd => {
+                let from = self
+                    .start_label
+                    .as_deref()
+                    .unwrap_or("structure");
+                format!(
+                    "Pipe run: click END from {from} (dia {:.2} ft, n {:.3}){hint}:",
+                    self.diameter, self.n
+                )
+            }
         }
     }
     fn needs_entity_pick(&self) -> bool {
+        false
+    }
+    fn needs_structure_point_pick(&self) -> bool {
         true
     }
-    fn on_entity_pick(&mut self, handle: Handle, pt: Vec3) -> CmdResult {
+    fn resolve_object_pick(&self, scene: &Scene, x: f64, y: f64) -> Option<ObjectPickHit> {
+        let pick = preview::structure_under_cursor(scene, x, y, false)?;
+        Some(ObjectPickHit {
+            handle: pick.handle,
+            x: pick.x,
+            y: pick.y,
+            label: pick.label(),
+        })
+    }
+    fn object_pick_hover_previews(&self, scene: &Scene, cursor: Vec3) -> Vec<WireModel> {
+        preview::structure_acquire_previews(scene, cursor, false)
+    }
+    fn object_pick_miss_message(&self) -> &'static str {
+        "No storm structure near click — move closer or press Enter for nearest."
+    }
+    fn set_acquisition_hint(&mut self, hint: Option<&str>) {
+        self.acquire_hint = hint.map(str::to_string);
+    }
+    fn on_structure_pick(&mut self, handle: Handle, pt: Vec3) -> CmdResult {
         match self.step {
             PStep::PickStart => {
                 self.start_handle = Some(handle);
                 self.start_xy = (pt.x as f64, pt.y as f64);
+                self.start_label = self.acquire_hint.clone();
+                self.acquire_hint = None;
                 self.step = PStep::PickEnd;
                 CmdResult::NeedPoint
             }
             PStep::PickEnd => self.commit(handle, pt.x as f64, pt.y as f64),
         }
     }
+    fn on_preview_wires(&mut self, pt: Vec3) -> Vec<WireModel> {
+        if matches!(self.step, PStep::PickEnd) {
+            vec![preview::pipe_run_rubber_band(self.start_xy.0, self.start_xy.1, pt)]
+        } else {
+            vec![]
+        }
+    }
     fn on_point(&mut self, _pt: Vec3) -> CmdResult {
         CmdResult::NeedPoint
     }
@@ -197,7 +251,6 @@ mod tests {
 
     #[test]
     fn click_places_structure_with_defaults() {
-        // A click commits immediately (no typed values needed).
         let mut cmd = PlaceStructure::inlet();
         match cmd.on_point(Vec3::new(10.0, 20.0, 0.0)) {
             CmdResult::CommitAndExit(EntityType::Circle(c)) => {
@@ -212,20 +265,19 @@ mod tests {
     #[test]
     fn typed_values_are_captured_then_click_places() {
         let mut cmd = PlaceStructure::inlet();
-        assert!(cmd.on_text_input("104").is_none()); // invert -> rim
-        assert!(cmd.on_text_input("110").is_none()); // rim -> area
-        assert!(cmd.on_text_input("2.0").is_none()); // area -> C
-        assert!(cmd.on_text_input("0.8").is_none()); // C -> Ready
-        assert!(matches!(cmd.step, SStep::Ready));
+        assert!(cmd.on_text_input("104").is_none());
+        assert!(cmd.on_text_input("110").is_none());
+        assert!(cmd.on_text_input("2.0").is_none());
+        assert!(cmd.on_text_input("0.8").is_none());
         assert!(matches!(cmd.on_point(Vec3::ZERO), CmdResult::CommitAndExit(_)));
     }
 
     #[test]
     fn pipe_connects_two_structures_on_two_clicks() {
         let mut cmd = PlacePipe::new();
-        assert!(cmd.needs_entity_pick());
-        assert!(matches!(cmd.on_entity_pick(Handle::new(1), Vec3::new(0.0, 0.0, 0.0)), CmdResult::NeedPoint));
-        match cmd.on_entity_pick(Handle::new(2), Vec3::new(100.0, 0.0, 0.0)) {
+        assert!(cmd.needs_structure_point_pick());
+        assert!(matches!(cmd.on_structure_pick(Handle::new(1), Vec3::new(0.0, 0.0, 0.0)), CmdResult::NeedPoint));
+        match cmd.on_structure_pick(Handle::new(2), Vec3::new(100.0, 0.0, 0.0)) {
             CmdResult::CommitAndExit(EntityType::Line(l)) => {
                 assert_eq!(l.start.x, 0.0);
                 assert_eq!(l.end.x, 100.0);
@@ -235,4 +287,13 @@ mod tests {
             _ => panic!("expected CommitAndExit(Line)"),
         }
     }
-}
+
+    #[test]
+    fn pipe_preview_rubber_band_on_second_step() {
+        let mut cmd = PlacePipe::new();
+        cmd.on_structure_pick(Handle::new(1), Vec3::new(0.0, 0.0, 0.0));
+        let wires = cmd.on_preview_wires(Vec3::new(50.0, 0.0, 0.0));
+        assert_eq!(wires.len(), 1);
+        assert_eq!(wires[0].points.len(), 2);
+    }
+}

src/modules/storm_sewer/style.rs

@@ -0,0 +1,61 @@
+// Visual feedback for analysis results (surcharged pipes, flooded structures).
+
+use acadrust::types::Color;
+use acadrust::{EntityType, Handle};
+
+use stormsewer::network::Analysis;
+
+use super::data::DrawnNetwork;
+
+fn color_surcharged_pipe() -> Color {
+    Color::from_index(1)
+}
+
+fn color_flooded_struct() -> Color {
+    Color::from_index(6)
+}
+
+/// Build handle → color assignments from an analysis result.
+pub fn style_assignments(drawn: &DrawnNetwork, analysis: &Analysis) -> Vec<(Handle, Color)> {
+    let mut out = Vec::new();
+    for (handle, pr) in drawn.pipe_handles.iter().zip(analysis.pipes.iter()) {
+        if pr.surcharged {
+            out.push((*handle, color_surcharged_pipe()));
+        }
+    }
+    for (handle, nr) in drawn.node_handles.iter().zip(analysis.nodes.iter()) {
+        if nr.surcharge_to_surface {
+            out.push((*handle, color_flooded_struct()));
+        }
+    }
+    out
+}
+
+/// Apply handle → color assignments to drawing entities.
+pub fn apply_colors<'a>(
+    entities: impl Iterator<Item = &'a mut EntityType>,
+    assignments: &[(Handle, Color)],
+) -> usize {
+    let mut applied = 0usize;
+    for e in entities {
+        let h = e.common().handle;
+        if let Some((_, color)) = assignments.iter().find(|(handle, _)| *handle == h) {
+            e.common_mut().color = *color;
+            applied += 1;
+        }
+    }
+    applied
+}
+
+/// Color-code pipes and structures from an analysis result.
+pub fn apply_analysis_style<'a>(
+    entities: impl Iterator<Item = &'a mut EntityType>,
+    drawn: &DrawnNetwork,
+    analysis: &Analysis,
+) -> (usize, usize) {
+    let assignments = style_assignments(drawn, analysis);
+    let surcharged = analysis.pipes.iter().filter(|p| p.surcharged).count();
+    let flooded = analysis.nodes.iter().filter(|n| n.surcharge_to_surface).count();
+    let _ = apply_colors(entities, &assignments);
+    (surcharged, flooded)
+}

src/plugin/host.rs

@@ -0,0 +1,17 @@
+// Plugin traits — HostSession lives in `app::plugin_host` (same-crate field access).
+
+pub(crate) use crate::app::plugin_host::HostSession;
+
+use crate::modules::CadModule;
+
+use super::manifest::PluginManifest;
+
+/// Add-on package entry point (phase 1: in-tree, in-process).
+///
+/// One `PluginRegistration` per package — ribbon tab, manifest, and command
+/// dispatch are owned here. See `docs/plugin-architecture.md`.
+pub trait BuiltinPlugin: Send + Sync {
+    fn manifest(&self) -> &'static PluginManifest;
+    fn ribbon(&self) -> Box<dyn CadModule>;
+    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool;
+}

src/plugin/manifest.rs

@@ -0,0 +1,32 @@
+// Plugin identity and capability declaration.
+
+/// Host plugin API version. Bump when HostApi breaks compatibility.
+pub const API_VERSION: u32 = 1;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct ApiVersion {
+    pub major: u32,
+}
+
+impl ApiVersion {
+    pub const CURRENT: Self = Self { major: API_VERSION };
+
+    pub fn is_compatible_with(host: ApiVersion) -> bool {
+        Self::CURRENT.major == host.major
+    }
+}
+
+/// Static metadata every plugin supplies at registration time.
+/// Keep fields in sync with `plugin.toml` beside the package.
+#[derive(Clone, Copy, Debug)]
+pub struct PluginManifest {
+    pub id: &'static str,
+    pub name: &'static str,
+    pub version: &'static str,
+    pub description: &'static str,
+    pub api_version: ApiVersion,
+    /// Sort key for add-on ribbon tabs (lower = further left among plugins).
+    pub ribbon_order: i32,
+    pub xdata_apps: &'static [&'static str],
+    pub command_prefixes: &'static [&'static str],
+}

src/plugin/mod.rs

@@ -0,0 +1,13 @@
+// Open CAD Studio plugin runtime (phase 1: built-in, in-process).
+//
+// Generic host only — no domain logic. See `docs/plugin-architecture.md`.
+// Domain plugins (e.g. storm_sewer) live under `src/modules/<name>/` and
+// register here via `inventory::submit!(PluginRegistration { … })`.
+
+pub mod host;
+pub mod manifest;
+pub mod registry;
+
+pub use registry::all_ribbon_modules;
+pub(crate) use host::BuiltinPlugin;
+pub(crate) use registry::try_dispatch;

src/plugin/registry.rs

@@ -0,0 +1,42 @@
+// Compile-time plugin registry via `inventory`.
+
+use super::host::{BuiltinPlugin, HostSession};
+use crate::app::OpenCADStudio;
+use crate::modules::{registry as core_registry, CadModule};
+
+pub struct PluginRegistration {
+    pub construct: fn() -> Box<dyn BuiltinPlugin>,
+}
+
+inventory::collect!(PluginRegistration);
+
+/// Construct every registered built-in plugin (once per process).
+pub fn all_plugins() -> Vec<Box<dyn BuiltinPlugin>> {
+    inventory::iter::<PluginRegistration>
+        .into_iter()
+        .map(|r| (r.construct)())
+        .collect()
+}
+
+/// Core ribbon tabs plus add-on tabs (sorted by `manifest.ribbon_order`).
+pub fn all_ribbon_modules() -> Vec<Box<dyn CadModule>> {
+    let mut core = core_registry::all_modules();
+    let mut addons: Vec<(i32, Box<dyn CadModule>)> = all_plugins()
+        .into_iter()
+        .map(|p| (p.manifest().ribbon_order, p.ribbon()))
+        .collect();
+    addons.sort_by_key(|(order, _)| *order);
+    core.extend(addons.into_iter().map(|(_, ribbon)| ribbon));
+    core
+}
+
+/// Try each plugin until one handles `cmd`. Returns true if handled.
+pub(crate) fn try_dispatch(app: &mut OpenCADStudio, tab: usize, cmd: &str) -> bool {
+    let mut host = HostSession::new(app, tab);
+    for plugin in all_plugins() {
+        if plugin.dispatch(&mut host, cmd) {
+            return true;
+        }
+    }
+    false
+}

src/scene/wire_model.rs

@@ -79,6 +79,9 @@ impl WireModel {
     pub const WHITE: [f32; 4] = [1.00, 1.00, 1.00, 1.0];
     pub const CYAN: [f32; 4] = [0.25, 0.85, 1.00, 1.0];
     pub const SELECTED: [f32; 4] = [0.15, 0.55, 1.00, 1.0];
+    /// Civil 3D-style object acquisition highlight (structure / catchment hover).
+    pub const PICK_HIGHLIGHT: [f32; 4] = [0.95, 0.50, 0.08, 1.0];
+    pub const PICK_HIGHLIGHT_DIM: [f32; 4] = [0.95, 0.50, 0.08, 0.45];
     /// Sentinel AABB that never rejects any snap query.
     pub const UNBOUNDED_AABB: [f32; 4] = [
         f32::NEG_INFINITY,

src/snap/mod.rs

@@ -30,6 +30,8 @@ pub enum SnapType {
     ApparentIntersection,
     Parallel,
     Grid,
+    /// C3D-style object acquisition (storm structure / network pick) — orange marker.
+    ObjectPick,
 }
 
 /// Ordered list used by the popup and snap engine.

src/ui/overlay.rs

@@ -500,18 +500,37 @@ impl canvas::Program<Message> for SelectionCanvas {
 
         // ── Snap marker ───────────────────────────────────────────────────
         if let Some((sp, snap_type)) = self.snap {
-            let yellow = Color {
-                r: 1.0,
-                g: 0.9,
-                b: 0.1,
-                a: 1.0,
+            let (r, g, b) = if snap_type == SnapType::ObjectPick {
+                (0.95_f32, 0.50, 0.08) // C3D-style orange object snap
+            } else {
+                (1.0, 0.9, 0.1) // classic yellow OSNAP
             };
+            let marker = Color { r, g, b, a: 1.0 };
             let stroke = canvas::Stroke {
-                width: 1.5,
-                style: canvas::Style::Solid(yellow),
+                width: if snap_type == SnapType::ObjectPick { 2.0 } else { 1.5 },
+                style: canvas::Style::Solid(marker),
                 ..Default::default()
             };
             match snap_type {
+                SnapType::ObjectPick => {
+                    // Target box + center dot (Civil 3D acquisition glyph).
+                    let h = 7.0_f32;
+                    let rect = canvas::Path::rectangle(
+                        Point::new(sp.x - h, sp.y - h),
+                        Size::new(h * 2.0, h * 2.0),
+                    );
+                    frame.stroke(&rect, stroke.clone());
+                    let r = 3.0_f32;
+                    frame.fill(
+                        &canvas::Path::circle(sp, r),
+                        Color {
+                            r: 0.95,
+                            g: 0.50,
+                            b: 0.08,
+                            a: 0.85,
+                        },
+                    );
+                }
                 SnapType::Endpoint => {
                     let h = 5.0_f32;
                     let rect = canvas::Path::rectangle(
@@ -655,7 +674,7 @@ impl canvas::Program<Message> for SelectionCanvas {
                     let r = 1.4_f32;
                     for k in [-7.0_f32, 0.0, 7.0] {
                         let dot = canvas::Path::circle(Point::new(sp.x + k, sp.y), r);
-                        frame.fill(&dot, yellow);
+                        frame.fill(&dot, marker);
                     }
                 }
                 SnapType::Parallel => {

src/ui/ribbon/mod.rs

@@ -13,8 +13,8 @@ use iced::widget::{button, column, container, mouse_area, row, scrollable, svg,
 use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
 
 use crate::app::Message;
-use crate::modules::registry;
 use crate::modules::{CadModule, IconKind, RibbonItem};
+use crate::plugin::all_ribbon_modules;
 use crate::ui::properties::{color_picker_dropdown, lw_options, LinetypeItem};
 
 mod widgets;
@@ -67,7 +67,7 @@ pub struct LayerInfo {
 impl Ribbon {
     pub fn new() -> Self {
         Self {
-            modules: registry::all_modules(),
+            modules: all_ribbon_modules(),
             active: 0,
             active_tool: None,
             wireframe: false,