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/renderers/node-renderer.tsx

@@ -12,6 +12,8 @@ import { RoofSegmentRenderer } from './roof-segment/roof-segment-renderer'
 import { ScanRenderer } from './scan/scan-renderer'
 import { SiteRenderer } from './site/site-renderer'
 import { SlabRenderer } from './slab/slab-renderer'
+import { StairRenderer } from './stair/stair-renderer'
+import { StairSegmentRenderer } from './stair-segment/stair-segment-renderer'
 import { WallRenderer } from './wall/wall-renderer'
 import { WindowRenderer } from './window/window-renderer'
 import { ZoneRenderer } from './zone/zone-renderer'
@@ -35,6 +37,8 @@ export const NodeRenderer = ({ nodeId }: { nodeId: AnyNode['id'] }) => {
       {node.type === 'zone' && <ZoneRenderer node={node} />}
       {node.type === 'roof' && <RoofRenderer node={node} />}
       {node.type === 'roof-segment' && <RoofSegmentRenderer node={node} />}
+      {node.type === 'stair' && <StairRenderer node={node} />}
+      {node.type === 'stair-segment' && <StairSegmentRenderer node={node} />}
       {node.type === 'scan' && <ScanRenderer node={node} />}
       {node.type === 'guide' && <GuideRenderer node={node} />}
     </>