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
The original implementation used spread ({ ...node }) which only shallow-copies.
Nested objects (asset, polygon, position tuples, metadata) would share references
between the original and cloned scene, meaning mutations to the clone could
corrupt the original.
Also removed resolveChildRefId helper — children arrays in the schema are always
string IDs, so the object-with-id handling was dead code.
