保持传播多区域结果为单个遮罩

- 后端传播落库时将同一 seed 在同一目标帧的多个不连通 polygon 保存到同一 annotation
- 同步任务传播和兼容同步传播接口的多 polygon 保存逻辑
- 传播结果 bbox 改为覆盖全部不连通 polygon，并保留多 polygon scores 与 holes
- 前端回显单条多 polygon annotation 时使用组合 bbox 和真实 polygon 面积
- 补充后端传播 worker 回归测试，验证不连通结果只生成一个 annotation
- 补充前端 API 回归测试，验证多 polygon annotation 回显为一个 mask
- 更新项目指南和设计冻结文档

src/lib/api.test.ts

@@ -693,6 +693,42 @@ describe('api client contracts', () => {
     }));
   });
 
+  it('restores disconnected polygons from one saved annotation as one mask with a combined bbox', async () => {
+    const { annotationToMask } = await import('./api');
+    const frame = { id: '5', projectId: '9', index: 0, url: '/frame.jpg', width: 100, height: 100 };
+
+    const hydrated = annotationToMask({
+      id: 44,
+      project_id: 9,
+      frame_id: 5,
+      template_id: null,
+      mask_data: {
+        polygons: [
+          [[0.1, 0.1], [0.2, 0.1], [0.2, 0.2], [0.1, 0.2]],
+          [[0.7, 0.7], [0.9, 0.7], [0.9, 0.9], [0.7, 0.9]],
+        ],
+        label: '多区域',
+        color: '#22c55e',
+        source: 'sam2.1_hiera_tiny_propagation',
+      },
+      points: null,
+      bbox: null,
+      created_at: 'created',
+      updated_at: 'updated',
+    }, frame);
+
+    expect(hydrated).toEqual(expect.objectContaining({
+      id: 'annotation-44',
+      pathData: 'M 10 10 L 20 10 L 20 20 L 10 20 Z M 70 70 L 90 70 L 90 90 L 70 90 Z',
+      segmentation: [
+        [10, 10, 20, 10, 20, 20, 10, 20],
+        [70, 70, 90, 70, 90, 90, 70, 90],
+      ],
+      bbox: [10, 10, 80, 80],
+      area: 500,
+    }));
+  });
+
   it('preserves propagation metadata when saving edited geometry without persisting preview-only smoothing fields', async () => {
     const { buildAnnotationPayload } = await import('./api');
     const frame = { id: '5', projectId: '9', index: 0, url: '/frame.jpg', width: 100, height: 50 };

src/lib/api.ts

@@ -592,6 +592,12 @@ function polygonToBbox(points: number[][], width: number, height: number): [numb
   return [minX, minY, maxX - minX, maxY - minY];
 }
 
+function polygonsToBbox(polygons: number[][][], width: number, height: number): [number, number, number, number] {
+  const points = polygons.flat();
+  if (points.length === 0) return [0, 0, 0, 0];
+  return polygonToBbox(points, width, height);
+}
+
 function polygonAreaPixels(points: number[][], width: number, height: number): number {
   if (points.length < 3) return 0;
   let total = 0;
@@ -803,7 +809,7 @@ export function annotationToMask(annotation: SavedAnnotation, frame: Frame): Mas
   const segmentationPolygons = mergedGeometry.segmentationPolygons;
   const firstPolygon = segmentationPolygons[0];
   if (!firstPolygon || firstPolygon.length === 0) return null;
-  const bbox = polygonToBbox(firstPolygon, frame.width, frame.height);
+  const bbox = polygonsToBbox(segmentationPolygons, frame.width, frame.height);
   const classMetadata = annotation.mask_data?.class;
   const { polygons: _polygons, holes: _holes, label: _label, color: _color, class: _classMetadata, ...metadata } = annotation.mask_data || {};
   const restoredMetadata = {
@@ -828,7 +834,7 @@ export function annotationToMask(annotation: SavedAnnotation, frame: Frame): Mas
     segmentation: segmentationPolygons.map((polygon) => polygon.flatMap(([x, y]) => [x * frame.width, y * frame.height])),
     points: annotation.points?.map(([x, y]) => [x * frame.width, y * frame.height]),
     bbox,
-    area: bbox[2] * bbox[3],
+    area: segmentationPolygons.reduce((total, polygon) => total + polygonAreaPixels(polygon, frame.width, frame.height), 0),
     metadata: hasMetadata ? restoredMetadata : undefined,
   };
 }