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/ChoiceVisibility.cs

@@ -0,0 +1,17 @@
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>
+/// Wire value of <c>open</c> on a choice/Discover <c>selectCard</c>: whether the pick is revealed.
+/// The client emits it as <c>selectCardIsOpen ? 1 : 0</c> (<c>SendKeyActionDataManager.cs</c>);
+/// the node uses it to decide whether to strip the pick for the opponent (<c>Hidden</c> = strip).
+/// Serializes as the underlying int via
+/// <see cref="System.Text.Json.Serialization.JsonNumberEnumConverter{T}"/>.
+/// </summary>
+public enum ChoiceVisibility
+{
+    /// <summary>Hidden draw-to-hand pick — the chosen card stays secret until played.</summary>
+    Hidden = 0,
+
+    /// <summary>Visible board choice — the pick is revealed immediately.</summary>
+    Open = 1,
+}