Contract C2: a script hand-off is confirmed only when the script takes the
Desktop's bridge marker in its own name, not when the launcher exits 0. The
channel hand-off test mocked spawnUpdaterProcess with a script that never
claimed, so apply() waited out the 20 s claim window and correctly reported
updater-spawn-failed. The fake now takes the marker as a live foreign pid,
and the test releases it after each hand-off (the update finished) so the
next apply does not see a live foreign owner. Claim timeout lowered to 2 s.
(cherry picked from commit fb43b97f0f501a2693130b4d8828014a380ae211)
b5ce061f41