refactor(battle-xp): retire ArenaTwoPickConfig.ClassXpPerBattle for shared BattleXpService

- ArenaTwoPickService now delegates class-XP grants to IBattleXpService
  with BattleXpMode.ArenaTwoPick. Inline GrantClassXp/ResolveClassLevel
  helpers deleted.
- ClassXpPerBattle field removed from ArenaTwoPickConfig; TK2 XP now
  configurable via ArenaTwoPickXpPerWin/XpPerLoss overrides or the global
  BattleXpConfig defaults (100/25).
- Updated 5 test files (ArenaTwoPickService callers) for the new
  constructor arg. Added loss-XP assertion to Finish tests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.UnitTests/Services/ArenaTwoPickServiceTopTests.cs

@@ -82,6 +82,6 @@ public class ArenaTwoPickServiceTopTests
     {
         // GetTopAsync only uses _runs — every other dep can be null! because the test path
         // never touches them.
-        return new ArenaTwoPickService(runRepo, null!, null!, null!, null!, null!, null!, db);
+        return new ArenaTwoPickService(runRepo, null!, null!, null!, null!, null!, null!, null!, db);
     }
 }