修复同类多实例传播结果被清理

- 在传播结果写入前的空间清理中识别同一任务里的其它 seed，避免后续同类 seed 删除前面刚写入的传播结果。

- 保留旧结果替换逻辑：不属于本次其它 seed 的旧传播结果仍可按语义和空间重叠被新 seed 替换。

- 将同类多实例传播回归测试改为重叠输出场景，覆盖此前两个同类方块只保留一个的问题。

backend/services/propagation_task_runner.py

@@ -286,6 +286,39 @@ def _seed_identity_matches(mask_data: dict[str, Any], seed_key: str, seed: dict[
     return _legacy_seed_matches(mask_data, seed)
 
 
+def _seed_identity_markers(seed: dict[str, Any]) -> set[str]:
+    markers = {f"seed:{_seed_key(seed)}"}
+    source_annotation_id = seed.get("source_annotation_id")
+    if source_annotation_id is not None:
+        markers.add(f"annotation:{source_annotation_id}")
+    source_mask_id = seed.get("source_mask_id")
+    if source_mask_id:
+        markers.add(f"mask:{source_mask_id}")
+    return markers
+
+
+def _mask_identity_markers(mask_data: dict[str, Any]) -> set[str]:
+    markers: set[str] = set()
+    previous_seed_key = mask_data.get("propagation_seed_key")
+    if previous_seed_key:
+        markers.add(f"seed:{previous_seed_key}")
+    source_annotation_id = mask_data.get("source_annotation_id")
+    if source_annotation_id is not None:
+        markers.add(f"annotation:{source_annotation_id}")
+    source_mask_id = mask_data.get("source_mask_id")
+    if source_mask_id:
+        markers.add(f"mask:{source_mask_id}")
+    return markers
+
+
+def _payload_seed_identity_markers(payload: dict[str, Any]) -> set[str]:
+    markers: set[str] = set()
+    for step in payload.get("steps") or []:
+        seed = step.get("seed") or {}
+        markers.update(_seed_identity_markers(seed))
+    return markers
+
+
 def _is_propagation_annotation(annotation: Annotation, seed_key: str, seed: dict[str, Any]) -> bool:
     mask_data = annotation.mask_data or {}
     source = str(mask_data.get("source") or "")
@@ -327,11 +360,17 @@ def _delete_replaced_frame_annotations(
         .all()
     )
     deleted_count = 0
+    current_seed_markers = _seed_identity_markers(seed)
+    task_seed_markers = _payload_seed_identity_markers(payload)
     for annotation in previous_annotations:
         mask_data = annotation.mask_data or {}
         source = str(mask_data.get("source") or "")
         if not source.endswith("_propagation"):
             continue
+        mask_markers = _mask_identity_markers(mask_data)
+        # Keep sibling seeds in the same propagation task from deleting each other.
+        if mask_markers and mask_markers.isdisjoint(current_seed_markers) and not mask_markers.isdisjoint(task_seed_markers):
+            continue
         same_lineage = _seed_identity_matches(mask_data, seed_key, seed)
         same_manual_replacement = (
             _semantic_seed_matches(mask_data, seed)

backend/tests/test_ai.py

@@ -756,8 +756,8 @@ def test_propagation_task_runner_keeps_same_class_seeds_separate(client, db_sess
     ]
 
     output_by_source = {
-        7: [[0.10, 0.10], [0.20, 0.10], [0.20, 0.20]],
+        7: [[0.10, 0.10], [0.30, 0.10], [0.30, 0.30], [0.10, 0.30]],
-        8: [[0.70, 0.70], [0.80, 0.70], [0.80, 0.80]],
+        8: [[0.12, 0.12], [0.32, 0.12], [0.32, 0.32], [0.12, 0.32]],
     }
     task = ProcessingTask(
         task_type="propagate_masks",