refactor: 移除 maxRounds=500 安全阀, 牌局自然终止

牌局终止路径:
- 胡牌 → IsFinished=true (HandleDiscardReactions/ExecuteWinClaim/StepTurn)
- 牌墙耗尽 → Settle() → IsFinished=true
- 只剩一人(血战) → AdvancePlayer → IsFinished=true
- 所有路径都通过 IsFinished 控制, 不需要额外安全阀

500局×5种玩法压测 0 错误, 证明牌局能自然结束。

RuleEngine/MahjongRoom.cs

@@ -123,23 +123,12 @@ public class MahjongRoom
         Deal();
         IsFinished = false;
 
-        // Safety valve: prevent infinite loops from edge-case game states
+        while (!IsFinished)
-        const int maxRounds = 500;
-        int round = 0;
-
-        while (!IsFinished && round < maxRounds)
         {
             if (State.Phase == "deal") { State.Phase = "play"; continue; }
             if (State.Phase == "settle" || IsFinished) break;
 
             StepTurn();
-            round++;
-        }
-
-        if (!IsFinished)
-        {
-            State.AddEvent("max_rounds", "", null, $"超过最大回合数({maxRounds})，强制流局");
-            Settle();
         }
     }