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/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;
+}