fix: center bga ball labels

XplorePlane/Services/ImageProcessing/WizardOverlayRenderer.cs

@@ -70,16 +70,26 @@ namespace XplorePlane.Services.ImageProcessing
                 Canvas.SetTop(ellipse, ball.CenterY - radius);
                 canvas.Children.Add(ellipse);
 
+                // 编号放在焊球中心，使用白底黑字提升在深色焊球上的可读性。
+                var labelWidth = Math.Max(34, Math.Min(72, radius * 0.9));
+                const double labelHeight = 26;
                 var label = new TextBlock
                 {
                     Text = string.IsNullOrEmpty(ball.Label) ? $"#{ball.Id}" : ball.Label,
-                    FontSize = 12,
+                    Width = labelWidth,
+                    Height = labelHeight,
+                    FontSize = 18,
                     FontWeight = FontWeights.Bold,
-                    Foreground = color,
+                    Foreground = Brushes.Black,
+                    Background = Brushes.White,
+                    Opacity = 0.92,
+                    TextAlignment = TextAlignment.Center,
+                    VerticalAlignment = VerticalAlignment.Center,
+                    Padding = new Thickness(2, 0, 2, 0),
                     IsHitTestVisible = false
                 };
-                Canvas.SetLeft(label, ball.CenterX - radius);
+                Canvas.SetLeft(label, ball.CenterX - labelWidth / 2.0);
-                Canvas.SetTop(label, ball.CenterY + radius + 2);
+                Canvas.SetTop(label, ball.CenterY - labelHeight / 2.0);
                 canvas.Children.Add(label);
             }
         }