fix(track-a): fence canonical lease post-merge races

[blakinio]

Post-merge remediation for task OTC-20260815-track-a-live-runtime-lease-manager, based on main@3575cc0c0a0b4efbcd9fc860d3226002fe40e70f after PR #312 merged.

Material findings addressed

Two P1 review findings landed on merged PR #312:

1. guard-run could release the coordination flock if its parent died while the guarded child continued running.
2. time-sensitive operations sampled time before blocking on the flock, so a lease expiring during lock wait could still be accepted.

Fix

• locked() now yields the live flock FD;
• guard-run passes that FD to the guarded child, so the child cannot outlive the serialization lock if the guard parent is killed;
• acquire, renew, release, validate, status, and guard-run now sample time only after the flock is acquired;
• regression tests cover post-lock expiry for every time-sensitive operation and parent-killed/child-survives lock inheritance.

Focused validation

Temporary branch-only remediation run 31908781559, job 95070594733: SUCCESS. The workflow applied the patch, ran the full lease unit suite, ran py_compile, committed the result, and was then removed from the branch. Final changed paths are only the manager implementation, its unit tests, and the active task record.

No Tibia client/runtime process, production canonical state, PR #303/#309 path, Track B path, or owner-funded AI/API quota is touched.

PR #311 remains fail-closed until this remediation is independently reviewed, protected CI passes, and the fix reaches main.

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