refactor(battlenode): type MatchContext.ClassId as CardClass enum (§C)

Behavior-preserving; full solution builds, 1013 tests green.

ClassId is the one genuinely-closed set of the three flagged stringly fields, so it
becomes a CardClass enum (1..8). Wire stays "1".."8": producer casts
(CardClass)run.ClassId, ServerBattleFrames renders via CardClassWire.ToWireValue().
RankBattleController's AI-start path drops a fragile int.TryParse(...)?:-1 for (int)cast.

CharaId (free-form leader/skin id, e.g. "5000123") and CountryCode (open-ended account
data) stay string with proper XML docs; CountryCodes.Korea/Japan name the captured values.

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

SVSim.BattleNode/Bridge/CountryCodes.cs

@@ -0,0 +1,13 @@
+namespace SVSim.BattleNode.Bridge;
+
+/// <summary>
+/// Known values for <see cref="MatchContext.CountryCode"/>. NOT a closed set — the field is the
+/// account's region code copied verbatim from viewer data (any value, possibly empty), and the node
+/// never branches on it. These constants just name the values seen in the prod captures so test
+/// fixtures and docs aren't sprinkled with bare <c>"KOR"</c>/<c>"JPN"</c> literals.
+/// </summary>
+public static class CountryCodes
+{
+    public const string Korea = "KOR";
+    public const string Japan = "JPN";
+}