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/lib/materials.ts

@@ -35,8 +35,8 @@ export function createMaterial(material?: MaterialSchema): THREE.MeshStandardMat
 }
 
 export function createDefaultMaterial(
-  color: string = '#ffffff',
-  roughness: number = 0.9,
+  color = '#ffffff',
+  roughness = 0.9,
 ): THREE.MeshStandardMaterial {
   return new THREE.MeshStandardMaterial({
     color,
@@ -59,6 +59,7 @@ export const DEFAULT_WINDOW_MATERIAL = new THREE.MeshStandardMaterial({
 })
 export const DEFAULT_CEILING_MATERIAL = createDefaultMaterial('#f5f5dc', 0.95)
 export const DEFAULT_ROOF_MATERIAL = createDefaultMaterial('#808080', 0.85)
+export const DEFAULT_STAIR_MATERIAL = createDefaultMaterial('#ffffff', 0.9)
 
 export function disposeMaterial(material: THREE.Material): void {
   material.dispose()