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/dispatch.rs

@@ -0,0 +1,14 @@
+use crate::plugin::host::HostSession;
+
+use super::manifest::PLUGIN_ID;
+
+pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+    let _ = (host, PLUGIN_ID);
+    match cmd {
+        "MP_HELLO" => {
+            host.push_info("Hello from my_plugin");
+            true
+        }
+        _ => false,
+    }
+}