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/hooks/use-node-events.ts

@@ -19,6 +19,10 @@ import {
   type SiteNode,
   type SlabEvent,
   type SlabNode,
+  type StairEvent,
+  type StairNode,
+  type StairSegmentEvent,
+  type StairSegmentNode,
   type WallEvent,
   type WallNode,
   type WindowEvent,
@@ -40,6 +44,8 @@ type NodeConfig = {
   ceiling: { node: CeilingNode; event: CeilingEvent }
   roof: { node: RoofNode; event: RoofEvent }
   'roof-segment': { node: RoofSegmentNode; event: RoofSegmentEvent }
+  stair: { node: StairNode; event: StairEvent }
+  'stair-segment': { node: StairSegmentNode; event: StairSegmentEvent }
   window: { node: WindowNode; event: WindowEvent }
   door: { node: DoorNode; event: DoorEvent }
 }