Errors, Process Cleanup, and Shutdown

RuntimeError describes recoverable operating failures from sources, filtering, process operations, and full channels. The bounded error channel feeds a configurable hook. That hook may place a CriticalError into a one-assignment cell, explicitly upgrading severity.

Critical failure belongs to the worker set. A graceful exit closes the event channel; unexpected critical errors terminate the main task. When the action worker ends, the main task shuts down the remaining JoinSet workers.

Graceful process shutdown is a protocol:

1. send the configured signal to the process group;
2. install a monotonic grace timer;
3. continue admitting urgent/high controls;
4. convert timer expiry into forceful Stop or restart;
5. await and reap the child;
6. raise completion flags and delete the job.

The CLI escalates repeated quit requests from graceful signal, to immediate force-stop, to abort. Process completion is converted back into an event, so --exit-on-error and queued reruns re-enter the same serialized policy path.