支持中空mask编辑和传播保洞

- 前端按 polygonRingCounts 维护外圈/内洞 ring 分组，中空 mask 在调整多边形时显示内洞顶点和插点手柄。

- 保存与回显标注时将中空结构拆分为 mask_data.polygons 和 mask_data.holes，导入/普通 mask 共享同一编辑体验。

- 自动传播 seed 携带 holes，SAM 2 seed 栅格化时扣除内洞，避免中空 mask 以实心形式传播。

- 传播结果轮廓提取改为保留层级内洞，并在同步传播和 Celery 传播落库时写回 holes 与 hasHoles。

- 传播 seed 签名纳入 holes，并加固保存结果时 holes 与原始 polygon 索引对齐。

- 补充前端保存/回显、Canvas 内洞编辑和后端 SAM 2 hole 处理测试。

- 更新 AGENTS、接口契约、需求冻结、设计冻结和测试计划文档，移除中空结构未实现的旧描述。

backend/services/propagation_task_runner.py

@@ -207,6 +207,7 @@ def _seed_signature(seed: dict[str, Any]) -> str:
         return str(inherited_signature)
     signature_payload = {
         "polygons": seed.get("polygons") or [],
+        "holes": seed.get("holes") or [],
         "bbox": seed.get("bbox") or [],
         "points": seed.get("points") or [],
         "labels": seed.get("labels") or [],
@@ -458,13 +459,14 @@ def _save_propagated_annotations(
         if not include_source and frame.id == source_frame.id:
             continue
         result_polygons = frame_result.get("polygons") or []
+        result_holes = frame_result.get("holes") or []
         scores = frame_result.get("scores") or []
-        smoothed_polygons = [
-            _smooth_polygon(polygon, smoothing)
-            for polygon in result_polygons
+        prepared_polygons = [
+            (polygon_index, _smooth_polygon(polygon, smoothing))
+            for polygon_index, polygon in enumerate(result_polygons)
             if len(polygon) >= 3
         ]
-        cleanup_polygon = next((polygon for polygon in smoothed_polygons if len(polygon) >= 3), None)
+        cleanup_polygon = next((polygon for _polygon_index, polygon in prepared_polygons if len(polygon) >= 3), None)
         if cleanup_polygon is not None and frame.id not in cleaned_frame_ids:
             deleted_count += _delete_replaced_frame_annotations(
                 db,
@@ -475,15 +477,17 @@ def _save_propagated_annotations(
                 polygon=cleanup_polygon,
             )
             cleaned_frame_ids.add(int(frame.id))
-        for polygon_index, polygon in enumerate(smoothed_polygons):
+        for polygon_index, polygon in prepared_polygons:
             if len(polygon) < 3:
                 continue
+            hole_group = result_holes[polygon_index] if polygon_index < len(result_holes) and isinstance(result_holes[polygon_index], list) else []
             annotation = Annotation(
                 project_id=int(payload["project_id"]),
                 frame_id=frame.id,
                 template_id=template_id,
                 mask_data={
                     "polygons": [polygon],
+                    **({"holes": [hole_group], "hasHoles": True} if hole_group else {}),
                     "label": label,
                     "color": color,
                     "source": f"{model_id}_propagation",