time.time() - path.stat().st_mtime went negative under load when NTFS
stamped a write's mtime slightly ahead of the wall clock read a moment
later, making a ttl_hours=0 cache lookup serve stale data instead of
refetching. surfaced as a flaky failure in test_cached_json_refetches_once_the_entry_expires
during the full suite run before task 40's smoke test.
