feat: ISMCTS 4项优化 — 模拟终止+候选剪枝+SanDai2带对子

A: 模拟截断 MinPlays评估
  旧: 张数最少=赢(不知道手牌结构) → 4张散牌 vs 3张对子, 4张赢(错)
  新: MinPlays最少=赢 → 散牌minPlays=4 vs 对子minPlays=2, 对子赢(对)

C: 候选剪枝 fastRate<0.15
  低胜率候选直接丢弃(非pass), 节省模拟+排序时间
  全剪枝时自动回退不剪枝

E: SanDai2/SiDai 优先带对子
  旧: 带最小两张散牌
  新: 优先找对子带, 找不到才带散牌(对子本身是一手牌)

注: B(预测引导采样)回退 — PredictOpponent在每局800次模拟中重复调
  用导致1.6s/局, 在线性能不可接受

验证: 100局0错误 0.8s/局

PdkFriendServer/Logic/IsmctsBot.cs

@@ -164,16 +164,19 @@ namespace PdkFriendServer.Logic
                         if (SimulateToEnd(hand, view, play))
                             wins++;
 
-                    // 有前途的候选多模拟
                     double fastRate = (double)wins / fastSims;
+
+                    // 胜率太低 → 剪枝，不参与后续评估
+                    if (fastRate < 0.15 && allCandidates.Count > 3)
+                        continue;
+
+                    // 有前途的候选多模拟
                     int extraSims = fastRate > 0.3 ? simsPerCandidate - fastSims : 0;
                     for (int s = 0; s < extraSims; s++)
                         if (SimulateToEnd(hand, view, play))
                             wins++;
 
                     double totalRate = (double)wins / (fastSims + extraSims);
-
-                    // 牌效率：出牌后剩余手牌的最少步数
                     int afterMin = HandOptimizer.MinPlays(
                         hand.Where(c =>
                             play.Ids == null || !play.Ids.Contains(c.ID)).ToArray());
@@ -181,6 +184,22 @@ namespace PdkFriendServer.Logic
                     scoredCandidates.Add((play, totalRate, afterMin, allCandidates[ci].isMulti));
                 }
 
+                // 如果所有候选都被剪枝，回退到不做剪枝，全部重新评估
+                if (scoredCandidates.Count == 0)
+                {
+                    for (int ci = 0; ci < allCandidates.Count; ci++)
+                    {
+                        int wins = 0;
+                        var play = allCandidates[ci].play;
+                        for (int s = 0; s < fastSims; s++)
+                            if (SimulateToEnd(hand, view, play)) wins++;
+                        double rate = (double)wins / fastSims;
+                        int am = HandOptimizer.MinPlays(
+                            hand.Where(c => play.Ids == null || !play.Ids.Contains(c.ID)).ToArray());
+                        scoredCandidates.Add((play, rate, am, allCandidates[ci].isMulti));
+                    }
+                }
+
                 // 4. 多牌加权
                 for (int i = 0; i < scoredCandidates.Count; i++)
                 {
@@ -460,7 +479,11 @@ namespace PdkFriendServer.Logic
                 }
                 current = current % 3 + 1;
             }
-            return hands[myPos - 1].Count <= hands[nextPos - 1].Count && hands[myPos - 1].Count <= hands[oppPos - 1].Count;
+            // 截断：用 MinPlays 评估而非比较张数
+            int myMin = HandOptimizer.MinPlays(hands[myPos - 1].ToArray());
+            int n1Min = HandOptimizer.MinPlays(hands[nextPos - 1].ToArray());
+            int n2Min = HandOptimizer.MinPlays(hands[oppPos - 1].ToArray());
+            return myMin <= n1Min && myMin <= n2Min;
         }
 
         // ---- 多牌先手枚举 ----
@@ -479,20 +502,48 @@ namespace PdkFriendServer.Logic
             var triples = GetMultiGroups(hand, 3);
             foreach (var t in triples)
             {
-                var rem = hand.Where(c => c.GameNum != t[0].GameNum).OrderBy(c => c.GameNum).ToList();
+                var rem = hand.Where(c => c.GameState == 1 && c.GameNum != t[0].GameNum).ToList();
                 if (rem.Count >= 2)
-                    AddCandidate(t.Concat(rem.Take(2)).ToList(), pos, candidates); // 三带二
+                {
+                    // 三带二：优先带对子（对子本身是一手牌），找不到对子才带两张最小散牌
+                    var pair = rem.GroupBy(c => c.GameNum)
+                        .Where(g => g.Count() >= 2)
+                        .OrderBy(g => g.Key)
+                        .FirstOrDefault();
+                    if (pair != null)
+                        AddCandidate(t.Concat(pair.Take(2)).ToList(), pos, candidates);
+                    else
+                        AddCandidate(t.Concat(rem.OrderBy(c => c.GameNum).Take(2)).ToList(), pos, candidates);
+                }
                 else
                     AddCandidate(t, pos, candidates);
             }
 
-            // 四带二/三
+            // 四带二/三：优先带对子
             var quads = GetMultiGroups(hand, 4);
             foreach (var q in quads)
             {
-                var rem = hand.Where(c => c.GameNum != q[0].GameNum).OrderBy(c => c.GameNum).ToList();
-                if (rem.Count >= 3) AddCandidate(q.Concat(rem.Take(3)).ToList(), pos, candidates);
-                else if (rem.Count >= 2) AddCandidate(q.Concat(rem.Take(2)).ToList(), pos, candidates);
+                var rem = hand.Where(c => c.GameState == 1 && c.GameNum != q[0].GameNum).ToList();
+                var pair = rem.GroupBy(c => c.GameNum)
+                    .Where(g => g.Count() >= 2)
+                    .OrderBy(g => g.Key)
+                    .FirstOrDefault();
+                if (rem.Count >= 3)
+                {
+                    if (pair != null)
+                        AddCandidate(q.Concat(pair.Take(2)).Concat(rem
+                            .Where(c => c.GameNum != pair.Key).OrderBy(c => c.GameNum).Take(1))
+                            .ToList(), pos, candidates);
+                    else
+                        AddCandidate(q.Concat(rem.OrderBy(c => c.GameNum).Take(3)).ToList(), pos, candidates);
+                }
+                else if (rem.Count >= 2)
+                {
+                    if (pair != null)
+                        AddCandidate(q.Concat(pair.Take(2)).ToList(), pos, candidates);
+                    else
+                        AddCandidate(q.Concat(rem.OrderBy(c => c.GameNum).Take(2)).ToList(), pos, candidates);
+                }
             }
 
             // 连对（复用 pairs，不重复 GetMultiGroups）