improve:白板翻页性能

Ink Canvas/MainWindow.xaml.cs

@@ -200,10 +200,8 @@ namespace Ink_Canvas
             inkCanvas.PreviewMouseDown += inkCanvas_PreviewMouseDown;
             inkCanvas.StylusDown += inkCanvas_StylusDown;
             inkCanvas.MouseRightButtonUp += InkCanvas_MouseRightButtonUp;
-            
-            // 注册橡皮擦操作结束事件（StylusUp 用于自动切换回批注；MouseUp 在 MW_ShapeDrawing.cs 的 inkCanvas_MouseUp 中会调用 HandleEraserOperationEnded）
+            // 注册橡皮擦操作结束事件（StylusUp 用于自动切换回批注；MouseUp 由 XAML 绑定触发，无需再单独注册）
             inkCanvas.StylusUp += inkCanvas_StylusUp;
-            inkCanvas.MouseUp += inkCanvas_MouseUp;
 
             // 初始化第一页Canvas
             var firstCanvas = new System.Windows.Controls.Canvas();

Ink Canvas/MainWindow_cs/MW_BoardControls.cs

@@ -512,11 +512,19 @@ namespace Ink_Canvas
             if (pageIndex == CurrentWhiteboardIndex)
             {
                 ClearStrokes(true);
-                if (CurrentWhiteboardIndex != WhiteboardTotalCount)
-                    for (var i = CurrentWhiteboardIndex; i <= WhiteboardTotalCount; i++)
+
+                var oldTotal = WhiteboardTotalCount;
+                if (CurrentWhiteboardIndex != oldTotal)
+                {
+                    for (var i = CurrentWhiteboardIndex; i < oldTotal; i++)
                         TimeMachineHistories[i] = TimeMachineHistories[i + 1];
+                }
                 else
+                {
                     CurrentWhiteboardIndex--;
+                }
+
+                TimeMachineHistories[oldTotal] = null;
                 WhiteboardTotalCount--;
                 RestoreStrokes();
             }