AdmissionFailed

The joiner's admit handshake failed terminally — it never entered a roster (joiner perspective only).

Terminal state — no further events follow. Room.broadcast and Room.sendTo become silent no-ops after this event.

Distinct from HostLost: HostLost means an already-admitted joiner lost its host; AdmissionFailed means admission never completed at all. Emitting it is the loud alternative to the pre-#1178 silent hang — without it, a dropped or refused Hello left the consumer waiting on Room.roster forever with no signal.

reason separates an active host refusal from silence — see AdmissionFailure.