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.

docs/plugin-template/PLUGIN.md

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+# My Plugin (`opencad.my_plugin`)
+
+## Commands
+
+| Command | Description |
+|---------|-------------|
+| `MP_HELLO` | Example command |
+
+## XDATA
+
+Document entity record layouts here when your plugin tags geometry.

docs/plugin-template/README.md

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+# Add-on plugin template
+
+Copy this folder to `src/modules/<your_module>/` and rename placeholders.
+
+## Quick start
+
+1. Copy files into `src/modules/my_plugin/`.
+2. Replace `MY_PLUGIN`, `my_plugin`, `opencad.my_plugin`, `MP_` throughout.
+3. Add `pub mod my_plugin;` to `src/modules/mod.rs`.
+4. `cargo build` — ribbon tab and commands register automatically.
+
+## Required files
+
+| File | Purpose |
+|------|---------|
+| `plugin.toml` | Metadata (QGIS-style); excludes dir from `build.rs` ribbon scan |
+| `manifest.rs` | Compile-time `PluginManifest` — keep in sync with `plugin.toml` |
+| `register.rs` | `inventory::submit!(PluginRegistration { … })` only |
+| `plugin.rs` | Thin `BuiltinPlugin` impl |
+| `dispatch.rs` | All command handlers |
+| `mod.rs` | `CadModule` ribbon + `CommandRegistration` for autocomplete |
+| `PLUGIN.md` | XDATA schemas and command reference |
+
+## Optional
+
+| File | Purpose |
+|------|---------|
+| `state.rs` | Per-document tab state via `host.ensure_plugin_state(PLUGIN_ID, …)` |
+| `crates/my_engine/` | Headless domain logic (no iced/acadrust) |
+
+See `docs/plugin-architecture.md` for the full spec.

docs/plugin-template/dispatch.rs

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+use crate::plugin::host::HostSession;
+
+use super::manifest::PLUGIN_ID;
+
+pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+    let _ = (host, PLUGIN_ID);
+    match cmd {
+        "MP_HELLO" => {
+            host.push_info("Hello from my_plugin");
+            true
+        }
+        _ => false,
+    }
+}

docs/plugin-template/manifest.rs

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+use crate::plugin::manifest::{ApiVersion, PluginManifest};
+
+pub const PLUGIN_ID: &str = "opencad.my_plugin";
+
+pub static MANIFEST: PluginManifest = PluginManifest {
+    id: PLUGIN_ID,
+    name: "My Plugin",
+    version: "0.1.0",
+    description: "Short description of what this add-on does",
+    api_version: ApiVersion::CURRENT,
+    ribbon_order: 60,
+    xdata_apps: &["MYPLUGIN_RECORD"],
+    command_prefixes: &["MP_"],
+};

docs/plugin-template/mod.rs

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+pub mod dispatch;
+pub mod manifest;
+pub mod plugin;
+pub mod register;
+
+use crate::modules::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
+
+inventory::submit!(crate::command::CommandRegistration {
+    names: &["MP_HELLO"]
+});
+
+pub struct MyPluginModule;
+
+impl CadModule for MyPluginModule {
+    fn id(&self) -> &'static str {
+        "my_plugin"
+    }
+
+    fn title(&self) -> &'static str {
+        "My Plugin"
+    }
+
+    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
+        vec![RibbonGroup {
+            title: "Tools",
+            tools: vec![RibbonItem::LargeTool(ToolDef {
+                id: "MP_HELLO",
+                label: "Hello",
+                icon: IconKind::Glyph("★"),
+                event: ModuleEvent::Command("MP_HELLO".to_string()),
+            })],
+        }]
+    }
+}

docs/plugin-template/plugin.rs

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+use crate::plugin::host::{BuiltinPlugin, HostSession};
+use crate::plugin::manifest::PluginManifest;
+
+use super::dispatch;
+use super::manifest;
+
+pub struct MyPlugin;
+
+impl BuiltinPlugin for MyPlugin {
+    fn manifest(&self) -> &'static PluginManifest {
+        &manifest::MANIFEST
+    }
+
+    fn ribbon(&self) -> Box<dyn crate::modules::CadModule> {
+        Box::new(super::MyPluginModule)
+    }
+
+    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool {
+        dispatch::handle(host, cmd)
+    }
+}

docs/plugin-template/plugin.toml

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+[plugin]
+id = "opencad.my_plugin"
+name = "My Plugin"
+version = "0.1.0"
+description = "Short description of what this add-on does"
+author = "Your Name"
+license = "GPL-3.0-only"
+
+[opencad]
+api_version = 1
+ribbon_order = 60
+command_prefixes = ["MP_"]
+xdata_apps = ["MYPLUGIN_RECORD"]

docs/plugin-template/register.rs

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+use super::plugin::MyPlugin;
+
+inventory::submit! {
+    crate::plugin::registry::PluginRegistration {
+        construct: || Box::new(MyPlugin),
+    }
+}