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/register.rs

@@ -0,0 +1,7 @@
+use super::plugin::MyPlugin;
+
+inventory::submit! {
+    crate::plugin::registry::PluginRegistration {
+        construct: || Box::new(MyPlugin),
+    }
+}