#478 Race Condition: Tool registry lazy load without lock

engine/tools/registry.py:205-211 _ensure_lazy_tools() has race condition: ```python if not self._lazy_loaded: # CHECK from engine.tools.workflow_branch_executor import ... self.register(...) self._lazy_loaded = True # USE ``` Race: Two threads can import and register simultaneously. FIX: Add threading.Lock with double-check locking.