Gate finding B05-1: the successor-forward branch of _hold_inbound_event
returned before self._accept_update(). The admission ContextVar is per
adapter instance, so the successor's own _accept_update() in its hold
path reads its own (empty) var and the predecessor's in-flight claim
stays unaccepted. _release_claim then skips _record_receipt, and the
update can be replayed after a restart even though it was handed off.
Accept on the retired adapter before forwarding, and deref the successor
weakref once.
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