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.rs

@@ -0,0 +1,21 @@
+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)
+    }
+}