refactor: _wildcards static→实例级 WildcardRegistry

Root cause: static HashSet跨测试/跨变体污染+硬编码名→编码映射

Changes:
- WildcardRegistry: 实例级wildcard管理(IsWildcard/CountWildcards/Clone)
- GameState: 新增Wildcards属性(含Clone)
- MahjongTile: 移除static_wildcards,IsWildcard仅查编码50-59
  +ToString(tile,wildcards)重载用于游戏上下文显示
- MeldsSolver: _wildcards字段→构造函数注入(WildcardRegistry?)
- PhaseMachine: _wildcards字段+构造注入
- Deal(): State.Wildcards.Clear()+AddRange替代static ConfigureWildcards
  +NameToEncoding查表替代switch硬编码
- RandomMahjongAI/HumanMahjongPlayer: state.Wildcards替代static调用
- HumanMahjongPlayer.CheckTing: 补wildcardCount参数(之前遗漏)
- ExecuteWinClaim: IsFinished=true→Settle()(之前Phase漏设)

Test: CoreTests清理ConfigureWildcards()→WildcardRegistry测试
Impact: 零static state→不同变体/测试完全隔离,未来支持随机宝牌

RuleEngine/AI/RandomMahjongAI.cs

@@ -40,7 +40,7 @@ public class RandomMahjongAI
         if (discards.Count > 0)
         {
             var hand = state.Hands.GetValueOrDefault(Name, new List<int>());
-            var nonWildTiles = hand.Where(t => !MahjongTile.IsWildcard(t)).ToList();
+            var nonWildTiles = hand.Where(t => !state.Wildcards.IsWildcard(t)).ToList();
             if (nonWildTiles.Count > 0)
                 return ("discard", nonWildTiles[_rng.Next(nonWildTiles.Count)]);
             // Only wildcards left — discard one