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/TurnStartHandler.cs

@@ -10,7 +10,7 @@ internal sealed class TurnStartHandler : IFrameHandler
     {
         // PvP: the active player's TurnStart{orderList} is dropped; the opponent receives {spin}
         // (spin=0 for the deterministic-turn slice) and self-generates its turn-open.
-        if (ctx.Type == BattleType.Pvp && ctx.BothSidesAfterReady())
+        if (ctx.BothSidesAfterReady())
         {
             var frame = ctx.Env with { Body = new OpponentTurnStartBody(Spin: BattleFrameDefaults.DeterministicTurnSpin) };
             return new[] { new DispatchRoute(ctx.Other, frame, Stock.Normal) };