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/app/plugin_host.rs

@@ -0,0 +1,94 @@
+// HostSession — plugin-facing API implemented inside `app` (private field access).
+
+use std::any::{Any, TypeId};
+
+use acadrust::{CadDocument, EntityType, Handle};
+
+use super::OpenCADStudio;
+use crate::command::CadCommand;
+
+/// Session adapter: one active document tab, command line, undo.
+pub(crate) struct HostSession<'a> {
+    app: &'a mut OpenCADStudio,
+    tab: usize,
+}
+
+impl<'a> HostSession<'a> {
+    pub(crate) fn new(app: &'a mut OpenCADStudio, tab: usize) -> Self {
+        Self { app, tab }
+    }
+
+    pub fn tab_index(&self) -> usize {
+        self.tab
+    }
+
+    pub fn document(&self) -> &CadDocument {
+        &self.app.tabs[self.tab].scene.document
+    }
+
+    pub fn document_mut(&mut self) -> &mut CadDocument {
+        &mut self.app.tabs[self.tab].scene.document
+    }
+
+    pub fn entities(&self) -> impl Iterator<Item = &EntityType> {
+        self.document().entities()
+    }
+
+    pub fn entities_mut(&mut self) -> impl Iterator<Item = &mut EntityType> {
+        self.document_mut().entities_mut()
+    }
+
+    pub fn add_entity(&mut self, entity: EntityType) -> Handle {
+        self.app.tabs[self.tab].scene.add_entity(entity)
+    }
+
+    pub fn bump_geometry(&mut self) {
+        self.app.tabs[self.tab].scene.bump_geometry();
+    }
+
+    pub fn push_undo(&mut self, label: &str) {
+        self.app.push_undo_snapshot(self.tab, label);
+    }
+
+    pub fn set_dirty(&mut self) {
+        self.app.tabs[self.tab].dirty = true;
+    }
+
+    pub fn push_info(&mut self, msg: &str) {
+        self.app.command_line.push_info(msg);
+    }
+
+    pub fn push_output(&mut self, msg: &str) {
+        self.app.command_line.push_output(msg);
+    }
+
+    pub fn push_error(&mut self, msg: &str) {
+        self.app.command_line.push_error(msg);
+    }
+
+    pub fn set_active_command(&mut self, cmd: Box<dyn CadCommand>) {
+        self.app.tabs[self.tab].active_cmd = Some(cmd);
+    }
+
+    pub fn plugin_state<T: Any + Send + Sync + 'static>(
+        &self,
+        plugin_id: &'static str,
+    ) -> Option<&T> {
+        self.app.tabs[self.tab].plugin_state(plugin_id, TypeId::of::<T>())
+    }
+
+    pub fn plugin_state_mut<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+    ) -> Option<&mut T> {
+        self.app.tabs[self.tab].plugin_state_mut(plugin_id, TypeId::of::<T>())
+    }
+
+    pub fn ensure_plugin_state<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+        init: impl FnOnce() -> T,
+    ) -> &mut T {
+        self.app.tabs[self.tab].ensure_plugin_state(plugin_id, init)
+    }
+}