Wire shelf + spawn placement polish: SFX, cursor, sidebar, selection

User-visible follow-ups after first running the Phase 2 spike.

Spawn tool now matches legacy UX:
- CursorSphere from @pascal-app/editor for the placement indicator
  (ring + line + tool-icon tooltip) — was a plain sphere mesh.
- Emits sfx:structure-build on commit + setTool(null) + setMode
  ('select') to exit build mode, matching legacy spawn-tool.

Shelf tool placement:
- Emits sfx:structure-build on commit.
- Cursor preview now shows top board + brackets (was just the top),
  matching what gets placed.

Shelf selectable from the 3D canvas:
- ShelfEvent type added to @pascal-app/core/events/bus.
- 'shelf' added to NodeConfig in useNodeEvents.
- ShelfRenderer wires `useNodeEvents(node, 'shelf')` handlers onto
  every mesh. Clicks/hovers now bubble through the editor's selection
  manager and update useViewer.selection.

Shelf appears in the sidebar:
- ShelfTreeNode component (mirrors spawn-tree-node's shape +
  selection/hover/rename wiring; lucide Layers icon).
- TreeNode dispatcher adds a `case 'shelf':` arm.

Framework changes:
- @pascal-app/editor exports CursorSphere alongside triggerSFX.
- @pascal-app/nodes now declares @pascal-app/editor as peer/dev dep.

Pre-existing typecheck errors in @pascal-app/editor (ceiling-tree-node,
fence-tree-node, slab-tree-node, scene.ts) are unchanged — present on
main and not introduced by this commit.

630 tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/nodes/package.json

@@ -24,6 +24,7 @@
   },
   "peerDependencies": {
     "@pascal-app/core": "^0.8.0",
+    "@pascal-app/editor": "^0.8.0",
     "@pascal-app/viewer": "^0.8.0",
     "@react-three/drei": "^10",
     "@react-three/fiber": "^9",
@@ -32,6 +33,7 @@
   },
   "devDependencies": {
     "@pascal-app/core": "^0.8.0",
+    "@pascal-app/editor": "^0.8.0",
     "@pascal-app/viewer": "^0.8.0",
     "@pascal/typescript-config": "*",
     "@types/bun": "^1.3.0",