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.EmulatedEntrypoint/Controllers/RankBattleController.cs

@@ -183,7 +183,7 @@ public sealed class RankBattleController : ControllerBase
                 Seed = seed,
                 Rank = 0,
                 BattlePoint = 0,
-                ClassId = int.TryParse(selfCtx.ClassId, out var cId) ? cId : -1,
+                ClassId = (int)selfCtx.ClassId,
                 CharaId = int.TryParse(selfCtx.CharaId, out var chId) ? chId : -1,
                 IsMasterRank = 0,
                 MasterPoint = 0,