Exchange-local admission/revocation guard reusable by protocol adapters.

Created only for an admitted exchange, never on dependency load. Rechecks the original grant through Policy with a hard callback deadline. Every exchange, including anonymous local discovery/enrollment, monitors its original admission pid. Anonymous exchanges acquire no grant and skip grant rechecks, but still terminate when their admission stops. The entire polling period, callback budget and interruption grace fit inside recheck_interval (at most five seconds). An adapter must supply an interruption function that closes blocked frontend I/O; the guard also ends the exchange process so its owned backend sockets close. No cleanup calls the host, opens a backend or cancels a native operation.