writeUpdateMarker unconditionally overwrites HERMES_HOME/.hermes-update-in-progress
before every hand-off. If the user retries "Update" while a prior updater is
still alive and parked (e.g. waiting for the desktop to exit), the retry's
pre-write clobbers the still-running updater's claim, so the older updater is
no longer recorded as the owner even though it's actively mutating the
checkout. A second updater can then run concurrently over the same tree.
Add updateHandoffConflict() to check for a live foreign marker owner before
spawning a new updater, and refuse the hand-off (surfacing an "update already
running" message) instead of overwriting the marker.
Ref: #75778
hermes-seaeye[bot]