Phase 5 Stage C: item floor plan — all 9 kinds now at Stage C

Item is the last Stage A-only kind to gain def.floorplan. Closes Stage C
across every registered kind.

Files added:
 - nodes/src/item/floorplan.ts: buildItemFloorplan inlines a self-
   contained parent-chain transform walker using `ctx.resolve`. Mirrors
   the legacy `getItemFloorplanTransform` math from editor/lib/floorplan/
   items.ts:
     * Wall parent: rotate item.position by wall's angle, anchor at
       wall.start, handle wall-side attachTo via wall.thickness offset.
     * Item parent (nested): recurse for parent's transform.
     * Level / slab / ceiling parent: item.position is level-local.
   Returns a rotated width × depth rectangle. asset.floorPlanUrl image
   overlay deferred for Phase 5 follow-up.

Files changed:
 - nodes/src/item/definition.ts: wires `floorplan: buildItemFloorplan`.
 - floorplan-panel.tsx: floorplanItemEntries useMemo short-circuits to
   [] when nodeRegistry.has('item'). Phase 6 deletes the entire useMemo.

Stage C coverage (all 9 registered kinds):
 shelf ✅ spawn ✅ fence ✅ slab ✅ ceiling ✅ wall ✅ door ✅ window ✅ item ✅

Next sessions: Stage B for door / window / wall (each large geometry
extraction), Stage D per kind (DragAction affordance ports), Stage E
(drop legacy panels), Phase 6 Stage F cleanup.

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

packages/editor/src/components/editor/floorplan-panel.tsx

@@ -8843,6 +8843,11 @@ export function FloorplanPanel() {
       })
   }, [movingFloorplanNodeRevision, spawns])
   const floorplanItemEntries = useMemo(() => {
+    // Item migrated to def.floorplan (Phase 5 Stage C). When registered,
+    // FloorplanRegistryLayer renders the item rectangle via the
+    // parent-chain transform walker; this legacy path short-circuits.
+    // Removed entirely in Phase 6 cleanup.
+    if (nodeRegistry.has('item')) return []
     const transformCache = new Map<string, SharedFloorplanNodeTransform | null>()
 
     return floorplanItems.flatMap((item) => {