Phase 4 follow-on: floor-plan interaction + def.toolHints + spawn floorplan builder

Three additions on top of the floor-plan registry contract:

1. def.toolHints + RegisteredToolHelper (registry contract for the
   shortcut hint panel)

   - New ToolHint type in core: { key, label } static array.
   - Added `def.toolHints?: ToolHint[]` to NodeDefinition.
   - New <RegisteredToolHelper hints={...}> in editor — same visual
     styling as WallHelper / ItemHelper but data-driven.
   - HelperManager: registry-first check before falling through to the
     hand-written per-tool switch. Per-tool helper files get deleted
     as their kind migrates `toolHints` in.
   - Shelf + spawn definitions ship toolHints today; wall ports in
     Phase 3 Milestone C alongside its tool/affordance port.

2. Floor-plan interaction layer (selection + drag-to-move)

   - <FloorplanRegistryLayer> now wraps each entry in an interactive
     <g>:
       * Click → useViewer.setSelection({ selectedIds: [id] }).
         Selection visual is a thicker accent-colored stroke applied
         via withSelectionStyle() recursion through the FloorplanGeometry
         tree — kinds don't author selection decoration.
       * Drag → imperative SVG transform during the gesture, single
         updateNode commit on pointerup. Same "smooth move" pattern as
         MoveRegistryNodeTool for 3D drag: no per-tick store update,
         no React re-render storm, no zundo bloat. Coordinate
         conversion via svg.getScreenCTM().inverse().
       * useScene.temporal.pause/resume brackets the gesture so one
         drag = one undo step.
   - Global pointermove / pointerup listeners so the gesture survives
     the cursor leaving the entry's bounding box (matches the legacy
     elevator-resize-drag and item-drag patterns in floorplan-panel).

3. Spawn floor-plan builder (deferred wiring)

   - buildSpawnFloorplan written but NOT wired on the definition —
     spawn already renders in the legacy floorplan-panel.tsx via
     `floorplanSpawnEntries`, and wiring def.floorplan now would
     double-render. The pure builder lives in nodes/src/spawn/
     floorplan.ts ready to wire when the legacy inline branch is
     removed (Phase 5 spawn-floorplan migration PR — same shape as
     wall's feature flag, but per kind inside the legacy panel).

Plan updated: floor-plan interaction section locks the click/drag
contract in, wall-floor-plan-as-legacy note flags everything advanced
the user sees today as legacy that ports alongside Milestone C.

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

packages/nodes/src/shelf/definition.ts

@@ -56,6 +56,10 @@ export const shelfDefinition: NodeDefinition<typeof ShelfNode> = {
 
   preview: () => import('./preview'),
   tool: () => import('./tool'),
+  toolHints: [
+    { key: 'Left click', label: 'Place shelf' },
+    { key: 'Esc', label: 'Cancel' },
+  ],
 
   presentation: {
     label: 'Shelf',