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/Dispatch/Handlers/LoadedHandler.cs

@@ -8,7 +8,7 @@ internal sealed class LoadedHandler : IFrameHandler
     public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
     {
         // case 3: Bot — silent (client populates opponent state from AIBattleStart HTTP data).
-        if (ctx.Type == BattleType.Bot && ctx.SenderPhase == HandshakePhase.AwaitingLoaded)
+        if (ctx.OpponentIsAckOnly && ctx.SenderPhase == HandshakePhase.AwaitingLoaded)
         {
             ctx.SenderPhase = HandshakePhase.AwaitingSwap;
             return Array.Empty<DispatchRoute>();