The stub polled for the lock with its own flock, competing with the
holder it had just started. The holder took the lock non-blockingly and
never retried, so a lost race killed it and left the lock free. The
notifier then saw no update in progress and sent the toast the test
asserts it withholds. Wait on the holder's own signal instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>