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/manifest.rs

@@ -0,0 +1,14 @@
+use crate::plugin::manifest::{ApiVersion, PluginManifest};
+
+pub const PLUGIN_ID: &str = "opencad.my_plugin";
+
+pub static MANIFEST: PluginManifest = PluginManifest {
+    id: PLUGIN_ID,
+    name: "My Plugin",
+    version: "0.1.0",
+    description: "Short description of what this add-on does",
+    api_version: ApiVersion::CURRENT,
+    ribbon_order: 60,
+    xdata_apps: &["MYPLUGIN_RECORD"],
+    command_prefixes: &["MP_"],
+};