PushAsync(TurnEnd|TurnEndFinal) fires FrameEmitted three times:
OpponentTurnStart + OpponentTurnEnd + OpponentJudge. Behaviour-identical
to the v1.2 case arm in BattleSession.ComputeResponses; just repackaged
as a participant. Other URIs are swallowed. Used by Phase 1 to preserve
v1.2 behaviour under the new abstraction; replaces the case-arm logic
in BattleSession in Task 7.
Silent participant for the Phase 3 Bot type. PushAsync swallows;
FrameEmitted never fires; RunAsync completes immediately. ViewerId is
the existing FakeOpponentViewerId const for consistency with scripted
lifecycle builders. Three tests lock the no-op contract.