fix: stabilize ping undo state and prevent negative ping steps

Assets/Scripts/Core/StepManager.cs

@@ -221,7 +221,7 @@ public class StepManager : MonoBehaviour
 
     public void DecrementPingStep()
     {
-        _pingStepIndex--;
+        _pingStepIndex = Mathf.Max(0, _pingStepIndex - 1);
         Debug.Log($"StepManager: Ping撤销 -> {_pingStepIndex}");
         OnStepChanged?.Invoke(_pingStepIndex);
     }

Assets/Scripts/Core/UndoManager.cs

@@ -30,6 +30,7 @@ public static class ActionHistory
     {
         if (History.Count == 0) return false;
         var item = History.Pop();
+        Debug.Log($"[ActionHistory] Undo {item.Type} handler={(item.Handler != null ? item.Handler.name : "null")}, remaining={History.Count}");
         
         if (item.Type == ActionType.Shose)
         {