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/mod.rs

@@ -0,0 +1,34 @@
+pub mod dispatch;
+pub mod manifest;
+pub mod plugin;
+pub mod register;
+
+use crate::modules::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
+
+inventory::submit!(crate::command::CommandRegistration {
+    names: &["MP_HELLO"]
+});
+
+pub struct MyPluginModule;
+
+impl CadModule for MyPluginModule {
+    fn id(&self) -> &'static str {
+        "my_plugin"
+    }
+
+    fn title(&self) -> &'static str {
+        "My Plugin"
+    }
+
+    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
+        vec![RibbonGroup {
+            title: "Tools",
+            tools: vec![RibbonItem::LargeTool(ToolDef {
+                id: "MP_HELLO",
+                label: "Hello",
+                icon: IconKind::Glyph("★"),
+                event: ModuleEvent::Command("MP_HELLO".to_string()),
+            })],
+        }]
+    }
+}