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).
