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);
 }

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[]>());
     }

SVSim.BattleNode/Wire/WireJsonOptions.cs

@@ -0,0 +1,24 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace SVSim.BattleNode.Wire;
+
+/// <summary>Shared System.Text.Json options for the bare-camelCase Socket.IO / Engine.IO wire:
+/// per-field <c>[JsonPropertyName]</c> casing (NOT EmulatedEntrypoint's snake_case policy), null
+/// fields omitted, and unattributed enums written as their name. Single-sourced here because
+/// <see cref="EngineIoHandshake"/> and <see cref="Protocol.MsgEnvelope"/> previously each built a
+/// byte-identical block in their own namespace — a drift hazard.</summary>
+internal static class WireJsonOptions
+{
+    public static readonly JsonSerializerOptions CamelCase = Create();
+
+    private static JsonSerializerOptions Create()
+    {
+        var opt = new JsonSerializerOptions
+        {
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+        };
+        opt.Converters.Add(new JsonStringEnumConverter());
+        return opt;
+    }
+}