UpdateMarkerGuard::acquire now takes the install root. Under the marker mutex, a dead marker is
kept (never compare-and-deleted) while the install's checkout kernel lock is held, mirroring
hermes_cli/update_lock.py: checkout_lock_path (common git dir or <root>/.hermes-update.lock) and
checkout_lock_held (POSIX flock probe; Windows LockFileEx on the byte at 1<<20 that msvcrt locks).
The refusal is Busy with a held owner (pid 0), and busy_update_message says the owner exited but a
process it started still holds the checkout. should_heal_self_marker_refusal no longer heals an
exit 2 while the checkout lock is held.
7d257fb619