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/snap/mod.rs

@@ -30,6 +30,8 @@ pub enum SnapType {
     ApparentIntersection,
     Parallel,
     Grid,
+    /// C3D-style object acquisition (storm structure / network pick) — orange marker.
+    ObjectPick,
 }
 
 /// Ordered list used by the popup and snap engine.