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/Lifecycle/ServerBattleFrames.cs

@@ -90,13 +90,14 @@ public static class ServerBattleFrames
 
     /// <summary>
     /// Compute the player's hand after a mulligan. For every idx in <paramref name="swapIndices"/>
-    /// that is currently in the hand, replace it with the next unused deck idx (starting at 4,
-    /// since 1..3 were dealt). Positions of kept cards are preserved.
+    /// that is currently in the hand, replace it with the next unused deck idx (the first idx past
+    /// the opening hand — <see cref="InitialHand"/> is 1-based and contiguous, so that's
+    /// <c>InitialHand.Length + 1</c>). Positions of kept cards are preserved.
     /// </summary>
     public static long[] ComputeHandAfterSwap(IReadOnlyList<long> swapIndices)
     {
         var hand = InitialHand.ToArray();
-        var nextDeckIdx = 4L;
+        var nextDeckIdx = (long)(InitialHand.Length + 1);
         for (var pos = 0; pos < hand.Length; pos++)
         {
             if (swapIndices.Contains(hand[pos]))
@@ -151,7 +152,7 @@ public static class ServerBattleFrames
             ViewerId: FakeOpponentViewerId,
             Uuid: WireConstants.ServerUuid,
             Bid: bid,
-            Try: 0,
+            RetryAttempt: 0,
             Cat: EmitCategory.Battle,
             PubSeq: null,
             PlaySeq: null,