A time-based grace kept a just-abandoned lease 'held', so back-to-back `hermes pm repair` runs
never collected the generation the previous run execv'd out of (e2e test_generation_gc red).
Rename+replace is out (msvcrt byte locks refuse renames on Windows), so the writer now re-checks
its lease path after taking the flock and retakes when a peer's prune unlinked it in between; the
pruner only unlinks while holding the lock, so a visible path after the lock is always ours.
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