feat: HUD显示所有玩家出牌记录 (DiscardHistory)

- GameState新增DiscardHistory字典 (per-player出牌记录)
- MahjongRoom.RecordDiscard统一记录出牌
- 人类模式HUD: 对手显示最近6张出牌
  例: AI-南: 10张手牌 | 已pung[..] | 出: 1万 3条 5筒

Demo/Program.cs

@@ -163,7 +163,13 @@ else
                             $"{m.Type}[{string.Join(",", m.Tiles.Where(t => t != -1).Select(MahjongTile.ToString))}]");
                         exposedStr = $" | 已{string.Join(" ", descs)}";
                     }
-                    Console.WriteLine($"    {p}: {hand.Count}张手牌{exposedStr}");
+                    var discardStr = "";
+                    if (room.State.DiscardHistory.TryGetValue(p, out var discards) && discards.Count > 0)
+                    {
+                        var recent = discards.TakeLast(6).Select(MahjongTile.ToString);
+                        discardStr = $" | 出: {string.Join(" ", recent)}";
+                    }
+                    Console.WriteLine($"    {p}: {hand.Count}张手牌{exposedStr}{discardStr}");
                 }
             }
             else