Reports whether handles from spawn will implement IWorkerHandle.onExit.

Answerable BEFORE a worker exists, which is the reason it sits here rather than being inferred from a handle: a consumer that must warn about — or configure around — undetectable worker death has to know at registration time, and the first spawn may be far later or never happen at all.

Optional so an existing custom host stays valid; a host that omits it is treated exactly as one reporting false.