A signal arriving between `mp.workerCmd = cmd` and `cmd.Start()`
returning was silently dropped (Process still nil), so a SIGTERM racing
a fresh fork never reached the new worker and systemd had to wait out
TerminateTimeout before sending SIGKILL. handleSignal now queues under
a new workerMux; fork drains the queue once Start succeeds.

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