Fixes AFT's internal file locks staying held after the owner releases them, because a forked child process can retain a duplicate descriptor until exec or exit. Maintenance operations like undo-history migration previously had to give up and retry; now locks are explicitly released via a new AdvisoryLockGuard when the protecting operation ends.
Applies the explicit-unlock guard to the backup namespace, session GC, manifest mutation, hook maintenance, root identity, check-cache, and broker-lease locks.
Adds Linux fork-based regression tests proving locks release even with a live duplicate descriptor.
No changes to migration policy or wait durations.
Confidence Score: 5/5
The PR appears safe to merge; no actionable issues were found.
Summary
Adds AdvisoryLockGuard to release file locks explicitly when protected work ends, even while a forked child retains a descriptor.
AFT file locks end when their operation ends, even after a fork.
