The short-list acknowledgment is a contract rule, so it gets a test that
says so rather than one that depends on a worker being slow.
acknowledge_replies carries the ipc-v1 section 5 sentence in its doc
comment and returns what the worker actually released;
acknowledge_lifecycle calls it, so bootstrap and the test exercise the
same path.
an_acknowledge_that_releases_nothing_is_not_a_failure drives the case
directly, once per execution mode: bootstrap releases every lifecycle
reply, the test asks for those ids again, the worker ignores them and
releases nothing, and the coordinator must accept the empty list, stay
unfenced, stay at its boundary and still play the next transition. It
fails if the equality check returns.
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