feat: stair system, scene graph utilities, read-only mode, viewer state improvements (#210)

Stair system (full stack):
- New StairNode + StairSegmentNode schemas with flights, landings, L/U-shapes
- StairSystem: geometry generation with throttled per-frame updates
- Stair tool, edit system, panels, tree node, and renderers
- Event bus types, scene registry, and command palette entries

Scene graph utilities:
- cloneLevelSubtree: deep-clone a level with remapped IDs
- forkSceneGraph: clone + strip scan/guide nodes for project forking

Core improvements:
- Read-only mode on scene store (blocks create/update/delete when locked)
- readOnly guards on node-actions and collection actions
- Upload store for scan/guide file upload handling

Viewer state:
- previewSelectedIds for box-select live preview
- hoverHighlightMode (default/delete) for delete-mode hover outline

packages/viewer/src/components/viewer/index.tsx

@@ -6,6 +6,7 @@ import {
   ItemSystem,
   RoofSystem,
   SlabSystem,
+  StairSystem,
   WallSystem,
   WindowSystem,
 } from '@pascal-app/core'
@@ -142,6 +143,7 @@ const Viewer: React.FC<ViewerProps> = ({
       <ItemSystem />
       <RoofSystem />
       <SlabSystem />
+      <StairSystem />
       <WallSystem />
       <WindowSystem />
       <ZoneSystem />