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/document.rs

@@ -9,6 +9,8 @@ use crate::ui::{LayerPanel, PropertiesPanel};
 use acadrust::tables::Ucs;
 use acadrust::{CadDocument, Handle};
 use iced;
+use std::any::{Any, TypeId};
+use std::collections::HashMap;
 use std::path::PathBuf;
 
 // ── Dynamic input ──────────────────────────────────────────────────────────
@@ -96,6 +98,8 @@ pub(super) struct DocumentTab {
     /// of the model-space shader. The scene is still constructed so the
     /// rest of the code can treat it as a normal tab when reading.
     pub(super) is_start: bool,
+    /// Per-plugin document state (`plugin::BuiltinPlugin` manifest id → state).
+    pub(super) plugin_state: HashMap<&'static str, Box<dyn Any + Send + Sync>>,
 }
 
 impl DocumentTab {
@@ -142,9 +146,41 @@ impl DocumentTab {
             active_mleader_style: "Standard".to_string(),
             last_synced_camera_gen: 0,
             is_start: false,
+            plugin_state: HashMap::new(),
         }
     }
 
+    pub(super) fn plugin_state<T: Any + Send + Sync + 'static>(
+        &self,
+        plugin_id: &'static str,
+        _type_id: TypeId,
+    ) -> Option<&T> {
+        self.plugin_state.get(plugin_id)?.downcast_ref::<T>()
+    }
+
+    pub(super) fn plugin_state_mut<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+        _type_id: TypeId,
+    ) -> Option<&mut T> {
+        self.plugin_state.get_mut(plugin_id)?.downcast_mut::<T>()
+    }
+
+    pub(super) fn ensure_plugin_state<T: Any + Send + Sync + 'static>(
+        &mut self,
+        plugin_id: &'static str,
+        init: impl FnOnce() -> T,
+    ) -> &mut T {
+        if !self.plugin_state.contains_key(plugin_id) {
+            self.plugin_state.insert(plugin_id, Box::new(init()));
+        }
+        self.plugin_state
+            .get_mut(plugin_id)
+            .expect("just inserted")
+            .downcast_mut::<T>()
+            .expect("plugin_id type mismatch")
+    }
+
     /// Welcome / Start tab. Carries a dummy Scene so the rest of the app
     /// can read tab state uniformly; the viewport renderer detects
     /// `is_start` and shows a welcome page instead.