fix: 人类玩家显示摸到的牌 (LastDrawnTile + 箭头高亮)

- GameState新增LastDrawnTile字段
- StepTurn摸牌时记录LastDrawnTile
- HumanMahjongPlayer显示'刚刚摸到:X'并用→←箭头标记新牌
- 庄家首轮LastDrawnTile清空

RuleEngine/MahjongRoom.cs

@@ -135,13 +135,16 @@ public class MahjongRoom
 
         // Phase 2: Draw card (skip for dealer on first turn — they already have 14)
         bool isDealerFirstTurn = (player == State.Dealer && State.RoundNumber == 0);
-        if (!isDealerFirstTurn)
+        if (isDealerFirstTurn)
+            State.LastDrawnTile = null;
+        else
         {
         if (State.Deck.Count > 0)
         {
             int drawn = State.Deck[^1];
             State.Deck.RemoveAt(State.Deck.Count - 1);
             State.Hands[player].Add(drawn);
+            State.LastDrawnTile = drawn;
             State.AddEvent("draw", player, drawn,
                 $"摸牌: {MahjongTile.ToString(drawn)}");