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/Services/MatchContextBuilder.cs

@@ -54,7 +54,7 @@ public class MatchContextBuilder : IMatchContextBuilder
 
         return new MatchContext(
             SelfDeckCardIds: deck,
-            ClassId: run.ClassId.ToString(),
+            ClassId: (CardClass)run.ClassId,
             CharaId: charaId,
             // Hardcoded v1; see spec §Deferred plumbing.
             CardMasterName: "card_master_node_10015",
@@ -104,7 +104,7 @@ public class MatchContextBuilder : IMatchContextBuilder
 
         return new MatchContext(
             SelfDeckCardIds: deckCardIds,
-            ClassId: deck.Class.Id.ToString(),
+            ClassId: (CardClass)deck.Class.Id,
             CharaId: charaId,
             CardMasterName: "card_master_node_10015",
             CountryCode: viewer.Info.CountryCode ?? string.Empty,