refactor(battle-node): de-magic wire flags and scattered constants

Quality pass from the 2026-06-04 BattleNode review (audit in the outer
repo). All changes are behavior-preserving — identical wire bytes,
verified by the full 1008-test suite staying green.

- Name scattered magic numbers: crypto key/IV lengths, outbound-sequencer
  base, WS receive buffer / EIO ping / SID length, polite-close timeout,
  upgrade-credential keys, battle-id digit math, deterministic-turn spin.
- resultCode = 1 -> (int)ReceiveNodeResultCode.Success across body records.
- Pong "3" -> EngineIoPacketType.Pong; remove dead NoOpBotParticipant.Touch
  (replace with #pragma warning disable CS0067).
- Wire-flag enums, serialized as numbers via JsonNumberEnumConverter:
  turnState -> TurnState{First,Second}, isSelf -> CardOwner{Opponent,Self},
  open -> ChoiceVisibility{Hidden,Open}.
- isOfficial / isInvoke -> bool / bool? via new NumericBoolJsonConverter
  (reads/writes 0/1; TDD'd). Scoped to the BattleNode wire boundary only;
  MatchContext and the HTTP/AI-start path stay int (AI-start uses -1 as a
  sentinel, so it is not boolean).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Protocol/Bodies/BattleStartBody.cs

@@ -3,11 +3,12 @@ using System.Text.Json.Serialization;
 namespace SVSim.BattleNode.Protocol.Bodies;
 
 public sealed record BattleStartBody(
-    [property: JsonPropertyName("turnState")] int TurnState,
+    [property: JsonPropertyName("turnState")]
+    [property: JsonConverter(typeof(JsonNumberEnumConverter<TurnState>))] TurnState TurnState,
     [property: JsonPropertyName("battleType")] int BattleType,
     [property: JsonPropertyName("selfInfo")] BattleStartSelfInfo SelfInfo,
     [property: JsonPropertyName("oppoInfo")] BattleStartOppoInfo OppoInfo,
-    [property: JsonPropertyName("resultCode")] int ResultCode = 1) : IMsgBody;
+    [property: JsonPropertyName("resultCode")] int ResultCode = (int)ReceiveNodeResultCode.Success) : IMsgBody;
 
 public sealed record BattleStartSelfInfo(
     [property: JsonPropertyName("rank")] string Rank,
@@ -18,6 +19,7 @@ public sealed record BattleStartSelfInfo(
 
 // Note: BattlePoint is int on the wire here (not string as on self) — matches the
 // captured prod frame at data_dumps/captures/battle-traffic_tk2_regular.ndjson.
+// The string-self / int-oppo split is INTENTIONAL; do NOT unify the two for "consistency".
 public sealed record BattleStartOppoInfo(
     [property: JsonPropertyName("rank")] string Rank,
     [property: JsonPropertyName("isMasterRank")] string IsMasterRank,