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/main.rs

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