Phase 2 of the job-finish robustness design (Part B, B1+B3).
B1 (worker liveness): a worker that stops reporting (heartbeat timeout,
already detected by check_worker_timeout_) or disconnects now has its RUN
jobs marked JOB_STAT_UNKWN|JOB_STAT_TRANSIENT (transient_since set, durable
WAL_JOB_UPDATE) instead of lingering as RUN forever. Marking happens
AFTER the worker lock is released (B5: avoids worker->jobmgr inversion
with the scheduler's jobmgr->worker order).
B3 (restart): persist_recover marks all recovered RUN jobs UNKWN+TRANSIENT
in-memory after replay — a controller restart leaves worker connectivity
unknown. In-memory only (no WAL append) because persist_recover may run
on a future standby that must never append.
Unit tests (red->green): worker-loss marks its RUN jobs unknown, other-host
jobs untouched, restart marks all RUN jobs. test_replay_apply_seam updated:
replay still drives facts to RUN, then B3 adds UNKWN+TRANSIENT.
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