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/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;