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/store/use-viewer.d.ts

@@ -12,6 +12,10 @@ type Outliner = {
 }
 type ViewerState = {
   selection: SelectionPath
+  previewSelectedIds: BaseNode['id'][]
+  setPreviewSelectedIds: (ids: BaseNode['id'][]) => void
+  hoverHighlightMode: 'default' | 'delete'
+  setHoverHighlightMode: (mode: 'default' | 'delete') => void
   hoveredId: AnyNode['id'] | ZoneNode['id'] | null
   setHoveredId: (id: AnyNode['id'] | ZoneNode['id'] | null) => void
   cameraMode: 'perspective' | 'orthographic'