refactor(battlenode): low-churn §B/§D/§E/§F quality cleanups

Behavior-preserving; 231 BattleNode tests green.

- §D: MsgEnvelope.Try -> RetryAttempt (drops keyword-escape; wire key stays "try");
  SocketIoFrame.AckResponse arg -> pubSeqEcho.
- §B: Gungnir.EmitInterval -> BattleNodeOptions.AliveEmitInterval (unused literal
  moved to its config home); deck-idx 4L -> InitialHand.Length + 1.
- §E: shared Wire.WireJsonOptions.CamelCase replaces the duplicated camelCase
  JsonSerializerOptions in EngineIoHandshake and MsgEnvelope.
- §F: do-NOT-consistency-fix polarity notes on TurnEndFinalHandler (From wins)
  and RetireKillHandler (From loses).

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

SVSim.BattleNode/Wire/SocketIoFrame.cs

@@ -158,10 +158,11 @@ public sealed class SocketIoFrame
             binaryAttachments: attachments);
     }
 
-    /// <summary>Build an ack response with a single int argument (the spec's pubSeq echo).</summary>
-    public static SocketIoFrame AckResponse(int ackId, int arg)
+    /// <summary>Build an ack response whose single argument echoes the inbound frame's pubSeq
+    /// (the client's ordered-delivery cursor — load-bearing, not a placeholder).</summary>
+    public static SocketIoFrame AckResponse(int ackId, int pubSeqEcho)
     {
-        var args = new JsonArray { arg };
+        var args = new JsonArray { pubSeqEcho };
         return new SocketIoFrame(
             SocketIoPacketType.Ack, ackId, 0, null, NodesToElements(args), Array.Empty<byte[]>());
     }