#3987 serve: worker enrollment resolves and creates the worker definition inside the serialized gateway transition tail

Follow-up from swamp-club#3984 (fix direction 2). WorkerGateway #handleEnroll runs the worker enroll method, including resolveOrCreateDefinition's lookup, lock and save, inside #recordTransition, so every gateway transition (other enrollments, worker_disconnected, set_status, dispatch's worker_busy) waits behind it. swamp-club#3984 makes a new-name lookup cheap (a name index of auto-definitions) but a burst still costs one or two directory listings per new name on the tail, about 0.5 s at 200k stored worker definitions locally. Proposal: after redeem, resolve or create the worker definition outside the tail (the auto-definition lock already serializes creation per name), and keep only the durable worker, token and lease writes on the tail. Must keep: no definition created for an unauthenticated request, already_connected and reconnect semantics, and closed-during-enrollment handling.