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/Lifecycle/ServerBattleFrames.cs

@@ -59,7 +59,7 @@ public static class ServerBattleFrames
                 SelfInfo: new BattleStartSelfInfo(
                     Rank: BattleFrameDefaults.PlayerRank,
                     BattlePoint: BattleFrameDefaults.PlayerBattlePoint,
-                    ClassId: selfCtx.ClassId,
+                    ClassId: selfCtx.ClassId.ToWireValue(),
                     CharaId: selfCtx.CharaId,
                     CardMasterName: selfCtx.CardMasterName),
                 OppoInfo: new BattleStartOppoInfo(
@@ -69,7 +69,7 @@ public static class ServerBattleFrames
                     IsMasterRank: "0",
                     BattlePoint: 0,
                     MasterPoint: "0",
-                    ClassId: oppoCtx.ClassId,
+                    ClassId: oppoCtx.ClassId.ToWireValue(),
                     CharaId: oppoCtx.CharaId,
                     CardMasterName: oppoCtx.CardMasterName)));