气泡测量：撤销功能、阈值说明优化、面板增加撤销按钮

XplorePlane/Views/ImageProcessing/BubbleMeasurePanel.xaml

@@ -21,7 +21,7 @@
         <Separator Margin="0,6" />
 
         <!-- 阈值 -->
-        <TextBlock Text="灰度阈值" Margin="0,0,0,4" />
+        <TextBlock Text="灰度阈值（与点击点的最大灰度差）" Margin="0,0,0,4" FontSize="11" Foreground="Gray" />
         <DockPanel>
             <TextBox x:Name="TbThreshold" DockPanel.Dock="Right" Width="45" Text="128"
                      VerticalContentAlignment="Center" Margin="6,0,0,0" />
@@ -51,6 +51,7 @@
 
         <!-- 操作按钮 -->
         <WrapPanel HorizontalAlignment="Center">
+            <Button Content="撤销" Padding="12,4" Margin="0,0,8,0" Click="Undo_Click" />
             <Button Content="清除标记" Padding="12,4" Margin="0,0,8,0" Click="ClearMask_Click" />
             <Button Content="完成" Padding="12,4" Click="Finish_Click" />
         </WrapPanel>

XplorePlane/Views/ImageProcessing/BubbleMeasurePanel.xaml.cs

@@ -72,6 +72,11 @@ namespace XplorePlane.Views.ImageProcessing
             TbResult.Text = "空隙率: --";
         }
 
+        private void Undo_Click(object sender, RoutedEventArgs e)
+        {
+            _canvas?.UndoBubble();
+        }
+
         private void Finish_Click(object sender, RoutedEventArgs e)
         {
             Close();