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/Bridge/BattleNodeOptions.cs

@@ -16,6 +16,13 @@ public sealed class BattleNodeOptions
     /// </summary>
     public TimeSpan WaitingRoomTimeout { get; set; } = TimeSpan.FromSeconds(60);
 
+    /// <summary>
+    /// Cadence of the server→client alive ("Gungnir") keepalive emit. The driving timer/loop
+    /// (to live on <see cref="Sessions.BattleSession"/>) is deferred in v1; this is its future
+    /// home so the interval isn't a magic literal stranded on the <c>Gungnir</c> body factory.
+    /// </summary>
+    public TimeSpan AliveEmitInterval { get; set; } = TimeSpan.FromSeconds(5);
+
     /// <summary>
     /// When true, <see cref="Sessions.Participants.RealParticipant"/> emits per-frame
     /// diagnostic logs at Information level: <c>[sio-in]</c> on every inbound msg/alive/hand