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/plugin/registry.rs

@@ -0,0 +1,42 @@
+// Compile-time plugin registry via `inventory`.
+
+use super::host::{BuiltinPlugin, HostSession};
+use crate::app::OpenCADStudio;
+use crate::modules::{registry as core_registry, CadModule};
+
+pub struct PluginRegistration {
+    pub construct: fn() -> Box<dyn BuiltinPlugin>,
+}
+
+inventory::collect!(PluginRegistration);
+
+/// Construct every registered built-in plugin (once per process).
+pub fn all_plugins() -> Vec<Box<dyn BuiltinPlugin>> {
+    inventory::iter::<PluginRegistration>
+        .into_iter()
+        .map(|r| (r.construct)())
+        .collect()
+}
+
+/// Core ribbon tabs plus add-on tabs (sorted by `manifest.ribbon_order`).
+pub fn all_ribbon_modules() -> Vec<Box<dyn CadModule>> {
+    let mut core = core_registry::all_modules();
+    let mut addons: Vec<(i32, Box<dyn CadModule>)> = all_plugins()
+        .into_iter()
+        .map(|p| (p.manifest().ribbon_order, p.ribbon()))
+        .collect();
+    addons.sort_by_key(|(order, _)| *order);
+    core.extend(addons.into_iter().map(|(_, ribbon)| ribbon));
+    core
+}
+
+/// Try each plugin until one handles `cmd`. Returns true if handled.
+pub(crate) fn try_dispatch(app: &mut OpenCADStudio, tab: usize, cmd: &str) -> bool {
+    let mut host = HostSession::new(app, tab);
+    for plugin in all_plugins() {
+        if plugin.dispatch(&mut host, cmd) {
+            return true;
+        }
+    }
+    false
+}