marker.sh::checkout_lock_held and marker.ps1::Test-CheckoutLockHeld answered
"not held" when the lock file exists but cannot be opened (root-owned after
`sudo hermes update`, or held open without sharing on Windows), while the
Desktop's own probes (checkout_held / WINDOWS_CHECKOUT_LOCK_PS) answer "held".
The script helper's reclaim then deleted a dead marker the Desktop's probe
keeps. Both scripts now fail closed on every open/probe error except a
missing file: reclaim needs a provably free checkout. Parity tests: the
POSIX probe with perl, python3 and flock(1), plus an end-to-end reclaim; the
Windows reclaim against a lock file held with no sharing (Windows CI lane).
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