declareAdmissionClosed

Publishes the admission-closed signal on the host's presence slot, replacing the "host" marker with an encoded form that carries the final voter set.

Call this once the host's admission loop reaches peerCount and exits — both in ReturnPolicy.FullMembership mode (synchronous path) and in ReturnPolicy.Quorum mode (background loop). The signal converges to every connected peer via the Quilter delta-exchange, where gameJoin observes it via admissionClosed.