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/Protocol/MsgEnvelope.cs

@@ -1,6 +1,6 @@
 using System.Text.Json;
 using System.Text.Json.Nodes;
-using System.Text.Json.Serialization;
+using SVSim.BattleNode.Wire;
 
 namespace SVSim.BattleNode.Protocol;
 
@@ -14,32 +14,21 @@ public sealed record MsgEnvelope(
     long ViewerId,
     string Uuid,
     string? Bid,
-    int Try,
+    int RetryAttempt,
     EmitCategory Cat,
     long? PubSeq,
     long? PlaySeq,
     IMsgBody Body)
 {
-    private static readonly JsonSerializerOptions Options = CreateOptions();
+    // Bare-camelCase wire serialization, single-sourced in Wire.WireJsonOptions (shared with
+    // EngineIoHandshake). Every wire key here is explicit via the manual ToJson layering below.
+    private static readonly JsonSerializerOptions Options = WireJsonOptions.CamelCase;
 
     private static readonly HashSet<string> ReservedEnvelopeKeys = new()
     {
         "uri", "viewerId", "uuid", "bid", "try", "cat", "pubSeq", "playSeq",
     };
 
-    private static JsonSerializerOptions CreateOptions()
-    {
-        var opt = new JsonSerializerOptions
-        {
-            // Wire-key casing is bare camelCase via per-field [JsonPropertyName] —
-            // NOT EmulatedEntrypoint's snake_case policy. The naming-policy line
-            // that was here previously was dead code (every wire key is explicit).
-            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
-        };
-        opt.Converters.Add(new JsonStringEnumConverter());
-        return opt;
-    }
-
     public static string ToJson(MsgEnvelope env)
     {
         // Envelope fields MUST come before body fields on the wire. The client's
@@ -51,7 +40,7 @@ public sealed record MsgEnvelope(
         result["uri"] = env.Uri.ToString();
         result["viewerId"] = env.ViewerId;
         result["uuid"] = env.Uuid;
-        result["try"] = env.Try;
+        result["try"] = env.RetryAttempt;
         result["cat"] = (int)env.Cat;
         if (env.Bid is not null) result["bid"] = env.Bid;
         if (env.PubSeq.HasValue) result["pubSeq"] = env.PubSeq.Value;
@@ -133,7 +122,7 @@ public sealed record MsgEnvelope(
         var viewerId = root.GetProperty("viewerId").GetInt64();
         var uuid = root.GetProperty("uuid").GetString()!;
         var bid = root.TryGetProperty("bid", out var bidEl) ? bidEl.GetString() : null;
-        var @try = root.TryGetProperty("try", out var tryEl) ? tryEl.GetInt32() : 0;
+        var retryAttempt = root.TryGetProperty("try", out var tryEl) ? tryEl.GetInt32() : 0;
         var cat = root.TryGetProperty("cat", out var catEl) ? (EmitCategory)catEl.GetInt32() : EmitCategory.Battle;
         var pubSeq = root.TryGetProperty("pubSeq", out var psEl) ? psEl.GetInt64() : (long?)null;
         var playSeq = root.TryGetProperty("playSeq", out var plsEl) ? plsEl.GetInt64() : (long?)null;
@@ -145,7 +134,7 @@ public sealed record MsgEnvelope(
             bodyDict[prop.Name] = ToObject(prop.Value);
         }
 
-        return new MsgEnvelope(uri, viewerId, uuid, bid, @try, cat, pubSeq, playSeq, new RawBody(bodyDict));
+        return new MsgEnvelope(uri, viewerId, uuid, bid, retryAttempt, cat, pubSeq, playSeq, new RawBody(bodyDict));
     }
 
     private static object? ToObject(JsonElement el) => el.ValueKind switch