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/stair-segment/stair-segment-renderer.tsx

@@ -0,0 +1,37 @@
+import { type StairSegmentNode, useRegistry, useScene } from '@pascal-app/core'
+import { useLayoutEffect, useMemo, useRef } from 'react'
+import type * as THREE from 'three'
+import { useNodeEvents } from '../../../hooks/use-node-events'
+import { createMaterial, DEFAULT_STAIR_MATERIAL } from '../../../lib/materials'
+
+export const StairSegmentRenderer = ({ node }: { node: StairSegmentNode }) => {
+  const ref = useRef<THREE.Mesh>(null!)
+
+  useRegistry(node.id, 'stair-segment', ref)
+
+  useLayoutEffect(() => {
+    useScene.getState().markDirty(node.id)
+  }, [node.id])
+
+  const handlers = useNodeEvents(node, 'stair-segment')
+
+  const material = useMemo(() => {
+    const mat = node.material
+    if (!mat) return DEFAULT_STAIR_MATERIAL
+    return createMaterial(mat)
+  }, [node.material, node.material?.preset, node.material?.properties, node.material?.texture])
+
+  return (
+    <mesh
+      material={material}
+      position={node.position}
+      ref={ref}
+      rotation-y={node.rotation}
+      visible={node.visible}
+      {...handlers}
+    >
+      {/* StairSystem will replace this geometry in the next frame */}
+      <boxGeometry args={[0, 0, 0]} />
+    </mesh>
+  )
+}