checkout_lock_held answered "free" when the lock file existed but could not be
opened. The hand-off scripts (marker.sh / marker.ps1) and the Desktop probes
(checkout_held, WINDOWS_CHECKOUT_LOCK_PS) answer "held" for the same file, so a
Python reader could reclaim a marker or admit a launch the others would park.
Only a missing lock file is free now; an existing file that cannot be opened
answers held. A lock call that fails outright (ENOLCK on NFS without lockd,
EOPNOTSUPP on some SMB shares) still answers free: nothing can hold such a
lock, and the interrupted-pull repair relies on that to run unguarded there.
Both callers that act on "held" are bounded (_reclaim_dead keeps the dead
marker; the acquire path waits its contention budget).
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