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/EngineIoHandshake.cs

@@ -12,11 +12,5 @@ public sealed record EngineIoHandshake(
     [property: JsonPropertyName("pingInterval")] int PingInterval,
     [property: JsonPropertyName("pingTimeout")] int PingTimeout)
 {
-    // Wire-key casing here is bare camelCase — NOT EmulatedEntrypoint's snake_case policy.
-    private static readonly JsonSerializerOptions Options = new()
-    {
-        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
-    };
-
-    public string ToJson() => JsonSerializer.Serialize(this, Options);
+    public string ToJson() => JsonSerializer.Serialize(this, WireJsonOptions.CamelCase);
 }