fix: Code Review完整修复 — AI chi/暗杠/加杠/chi位置

发现的6个缺口:
1. HandleDiscardReactions AI分支缺chi → 新增ExecuteAiChi
2. StepTurn不处理an_kong/bu_kong → 新增ExecuteAnKong/ExecuteBuKong
3. PhaseMachine.GetLegalActions chi误在自回合 → 移除(chi只能反应)
4. ScoreEngine.CheckFinish死代码 → 保留(无影响)
5. ExecutePungKong用Random.Shared → 改用_rng
6. HumanMahjongPlayer残留chiOpt → 移除

RuleEngine/Phase/PhaseMachine.cs

@@ -96,13 +96,6 @@ public class MahjongPhaseMachine
                 actions.Add(new ActionOption { Action = "win", Priority = 4 });
         }
 
-        // Chi
-        if (state.LastDiscard.HasValue && state.LastDiscardPlayer != playerId)
-        {
-            if (CanChiCheck(state, playerId, state.LastDiscard.Value))
-                actions.Add(new ActionOption { Action = "chi", Priority = 1 });
-        }
-
         actions.Add(new ActionOption { Action = "pass", Priority = 0 });
         return actions;
     }