poll() still calls a child that has exited but not been reaped alive, so a
node that ended itself with os._exit left its corpse on the idle queue and
the next call died on a broken pipe. Mark it the way the out-of-step paths
already do, and _release replaces it.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>