feat(battle-node): EchoHandler consumes Echo instead of relaying

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

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -50,7 +50,7 @@ public sealed class BattleSession
             [NetworkBattleUri.TurnStart] = new TurnStartHandler(),
             [NetworkBattleUri.Judge] = new JudgeHandler(),
             [NetworkBattleUri.PlayActions] = new PlayActionsHandler(),
-            [NetworkBattleUri.Echo] = forwardWhenReady,
+            [NetworkBattleUri.Echo] = new EchoHandler(),
             [NetworkBattleUri.TurnEndActions] = forwardWhenReady,
             [NetworkBattleUri.JudgeResult] = forwardWhenReady,
         };

SVSim.BattleNode/Sessions/Dispatch/Handlers/EchoHandler.cs

@@ -0,0 +1,8 @@
+namespace SVSim.BattleNode.Sessions.Dispatch.Handlers;
+
+/// <summary>Echo is the receiver's per-frame ack; the client has no inbound Echo handler, so the
+/// node consumes it (bullet-2 audit). Relaying would risk an echo->echo storm.</summary>
+internal sealed class EchoHandler : IFrameHandler
+{
+    public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx) => Array.Empty<DispatchRoute>();
+}