refactor(battlenode): key dispatch on OpponentIsAckOnly, drop per-frame BattleType switch

Behavior-identical; 231 BattleNode tests green with ZERO test changes.

The 10 handler arms no longer switch on BattleType:
- 4 Bot arms gate on the new FrameDispatchContext.OpponentIsAckOnly
  (Other is not IHasHandshakePhase) — the participant property the audit asked for.
- 6 relay arms drop the Type == Pvp guard; it was redundant with BothSidesAfterReady()
  (only a two-real-player session has both handshake phases). Its doc now records that.
- FrameDispatchContext.Type removed (+ the Type = Type in BuildContext). BattleSession.Type
  stays for the session-level drop cascade.

Zero test churn because the stubs already encode the split: FakeRealParticipant/ProbeParticipant
implement IHasHandshakePhase, the bot stub FakeParticipant doesn't, and NewBotSession uses it as
the opponent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -71,7 +71,7 @@ public sealed class BattleSession
         new()
         {
             A = A, B = B, From = from, Other = ReferenceEquals(from, A) ? B : A,
-            Env = env, Type = Type, BattleId = BattleId, State = _state,
+            Env = env, BattleId = BattleId, State = _state,
         };
 
     public BattleSession(string battleId, BattleType type, IBattleParticipant a, IBattleParticipant b,