Registers a listener for the worker's THREAD ENDING, however it ended — a clean self-termination included. This is distinct from onError, which reports a failure the worker survived long enough to report; a worker that simply stops raises no error at all.

Optional, and its absence is load-bearing information rather than a shortfall to work around: a host omits it when its runtime emits nothing when a worker ends, so a consumer MUST treat absence as "this runtime cannot tell me a worker died" and not as "no worker has died". Ask IWorkerHost.reportsExit before spawning if the answer changes what you configure.

Fires for a host-requested terminate() too on runtimes that implement it that way, so a consumer that terminates deliberately must track that itself rather than reading every exit as a crash.