RealParticipant gains _sessionFinished TCS + MarkSessionFinished /
AwaitSessionFinishedAsync. PvP first-arriver's handler awaits the
signal instead of calling self.RunAsync (which the session does
internally on the same instance — double-call would race the WS read).
BattleSession.RunAsync branches on Type: Pvp uses WhenAny + synthesize
BattleFinish(Win) to survivor + WhenAll(drain); Scripted/Bot keep
Phase 1's WhenAll-everything semantics. Disconnect cascade now drives
end-of-battle when a WS drops without a graceful Retire.
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