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/mod.rs

@@ -0,0 +1,13 @@
+// Open CAD Studio plugin runtime (phase 1: built-in, in-process).
+//
+// Generic host only — no domain logic. See `docs/plugin-architecture.md`.
+// Domain plugins (e.g. storm_sewer) live under `src/modules/<name>/` and
+// register here via `inventory::submit!(PluginRegistration { … })`.
+
+pub mod host;
+pub mod manifest;
+pub mod registry;
+
+pub use registry::all_ribbon_modules;
+pub(crate) use host::BuiltinPlugin;
+pub(crate) use registry::try_dispatch;