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/plugin.toml

@@ -0,0 +1,13 @@
+[plugin]
+id = "opencad.my_plugin"
+name = "My Plugin"
+version = "0.1.0"
+description = "Short description of what this add-on does"
+author = "Your Name"
+license = "GPL-3.0-only"
+
+[opencad]
+api_version = 1
+ribbon_order = 60
+command_prefixes = ["MP_"]
+xdata_apps = ["MYPLUGIN_RECORD"]