futures = {executor.submit(run_trajectory, t): t for t in trajectories}
for fut in as_completed(futures):
    try:
        collect(fut.result(timeout=120))     # per-trajectory watchdog
    except TimeoutError:
        mark(futures[fut], "timeout")         # record it, keep sweeping
executor.shutdown(wait=False, cancel_futures=True)  # never wait on a stuck thread