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/core/src/hooks/scene-registry/scene-registry.ts

@@ -18,6 +18,8 @@ export const sceneRegistry = {
     zone: new Set<string>(),
     roof: new Set<string>(),
     'roof-segment': new Set<string>(),
+    stair: new Set<string>(),
+    'stair-segment': new Set<string>(),
     scan: new Set<string>(),
     guide: new Set<string>(),
     window: new Set<string>(),