fix(editor): guard onShelfMove against null cursorGroupRef (#342)

Adds the missing `if (!cursorGroupRef.current) return` guard to onShelfMove. mitt listeners are registered for the tool's whole lifetime, so a shelf event can fire before the cursor group mounts or after teardown, leaving the ref null and throwing on `cursorGroupRef.current.rotation.y`.

Partial fix for the EDITOR-BC family: the other placement handlers (onGridMove/onWallMove/onItemMove/onCeilingMove and the keyboard/cancel paths) share the same null-ref exposure and still need the broader, correctly-scoped guard — tracked separately (PR #323 attempts this but needs rework).

packages/editor/src/components/tools/item/use-placement-coordinator.tsx

@@ -1147,6 +1147,9 @@ export function usePlacementCoordinator(config: PlacementCoordinatorConfig): Rea
 
     const onShelfMove = (event: ShelfEvent) => {
       has3DPointerDrivenMoveRef.current = true
+      // A shelf event can fire before the cursor group mounts or after
+      // teardown, leaving the ref null; bail before dereferencing it below.
+      if (!cursorGroupRef.current) return
       const ctx = getContext()
       if (ctx.state.surface !== 'shelf-surface') {
         // Cursor entered via a move event without an enter — try