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.UnitTests/BattleNode/Protocol/Bodies/BattleStartBodyTests.cs

@@ -1,6 +1,7 @@
 using System.Text.Json;
 using System.Text.Json.Nodes;
 using NUnit.Framework;
+using SVSim.BattleNode.Protocol;
 using SVSim.BattleNode.Protocol.Bodies;
 
 namespace SVSim.UnitTests.BattleNode.Protocol.Bodies;
@@ -12,7 +13,7 @@ public class BattleStartBodyTests
     public void Serializes_TopLevelFields_WithCorrectWireKeys()
     {
         var body = new BattleStartBody(
-            TurnState: 0, BattleType: 11,
+            TurnState: TurnState.First, BattleType: 11,
             SelfInfo: new BattleStartSelfInfo("10", "6270", "1", "1", "card_master_node_10015"),
             OppoInfo: new BattleStartOppoInfo("1", "0", 0, "0", "8", "8", "card_master_node_10015"));