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/FrameDispatchContext.cs

@@ -14,10 +14,17 @@ internal sealed class FrameDispatchContext
     internal required IBattleParticipant From { get; init; }
     internal required IBattleParticipant Other { get; init; }
     internal required MsgEnvelope Env { get; init; }
-    internal required BattleType Type { get; init; }
     internal required string BattleId { get; init; }
     internal required BattleSessionState State { get; init; }
 
+    /// <summary>The opponent is an AI-passive (ack-only) bot: it runs no handshake — no
+    /// <see cref="IHasHandshakePhase"/> — and receives no relayed frames (the client drives its own
+    /// AI; the server only acks). This is the participant property that replaces the per-handler
+    /// <c>BattleType.Bot</c> switch: the Bot dispatch arms gate on it. Its inverse — a live relay
+    /// peer — is what <see cref="BothSidesAfterReady"/> already implies (only real peers have a
+    /// handshake phase), so the relay arms need no separate opponent check.</summary>
+    internal bool OpponentIsAckOnly => Other is not IHasHandshakePhase;
+
     /// <summary>The dispatching participant's handshake phase (null for a non-IHasHandshakePhase
     /// participant, e.g. NoOpBot). Setting it advances the sender.</summary>
     internal HandshakePhase? SenderPhase
@@ -34,7 +41,10 @@ internal sealed class FrameDispatchContext
 
     /// <summary>BOTH participants have finished the handshake. Reads A/B (not From/Other) so the
     /// result is identical regardless of which side sent the frame. Contrast
-    /// <see cref="SenderIsAfterReady"/> (sender only).</summary>
+    /// <see cref="SenderIsAfterReady"/> (sender only). Only a live relay peer (real player) has a
+    /// handshake phase, so this can only be true in a two-real-player (PvP) session — the relay
+    /// dispatch arms gate on this instead of a <c>BattleType</c> check (an ack-only bot opponent,
+    /// <see cref="OpponentIsAckOnly"/>, can never satisfy it).</summary>
     internal bool BothSidesAfterReady() =>
         (A as IHasHandshakePhase)?.Phase == HandshakePhase.AfterReady &&
         (B as IHasHandshakePhase)?.Phase == HandshakePhase.AfterReady;