忽略完全位于图片外的切分标记矩形

矩形起点超出图片边界时直接返回 None，避免在边缘绘制伪影。

新增越界矩形回归测试。

apps/ai-game-creator-shell/src-tauri/src/ui_editor/commands/separation/marker.rs

@@ -119,8 +119,11 @@ fn clipped_rect(
     if width == 0 || height == 0 || w == 0 || h == 0 {
         return None;
     }
-    let x0 = x.min(width - 1);
+    if x >= width || y >= height {
-    let y0 = y.min(height - 1);
+        return None;
+    }
+    let x0 = x;
+    let y0 = y;
     let x1 = x.saturating_add(w).min(width).saturating_sub(1);
     let y1 = y.saturating_add(h).min(height).saturating_sub(1);
     (x0 <= x1 && y0 <= y1).then_some((x0, y0, x1, y1))
@@ -301,6 +304,12 @@ mod tests {
         assert_eq!(*image.get_pixel(1, 1), image::Rgba([1, 2, 3, 255]));
     }
 
+    #[test]
+    fn clipped_rect_ignores_rectangles_starting_outside_image() {
+        assert_eq!(clipped_rect(4, 0, 1, 1, 4, 4), None);
+        assert_eq!(clipped_rect(0, 4, 1, 1, 4, 4), None);
+    }
+
     #[test]
     fn text_mask_is_purple_before_green_frame() {
         let mut image = image::RgbaImage::from_pixel(8, 8, image::Rgba([1, 2, 3, 255]));