declareVacate

Declare that this voter is voluntarily leaving the session.

The host observes this via vacaters and immediately evicts the voter via RaftNode.changeMembership, without waiting the reconnect window. Call this from GameSession.leave before closing the session so the seat is freed promptly.

The vacate signal coexists with PRESENT_DECLARED: after calling this the replica's slot holds VACATE_DECLARED, overwriting PRESENT_DECLARED. The Quilter delivers the delta to the host.