fix: 避免自动传播重复叠加同源 mask

Bugfix：自动传播 worker 改为在本次目标帧段内按 seed 来源、方向、权重和签名查找旧传播结果；未修改且目标帧已覆盖时直接跳过，不再重复跑 SAM 造成 mask 堆叠。

Bugfix：同一 seed 被编辑、目标帧段只部分覆盖或切换 SAM 2.1 权重时，worker 会先删除本次目标帧段内同源旧自动传播标注，再重新传播。

Bugfix：未编辑的自动传播结果再次作为参考 seed 时会继承原始 propagation_seed_signature；编辑后的传播结果只保留 source_annotation_id/source_mask_id lineage，不继承旧签名，从而触发重传路径。

Bugfix：后端传播签名增加 canonical rounding，减少浮点精度细微变化导致未编辑 mask 被误判为已修改。

功能调整：清空片段遮罩改成与自动传播一致的时间轴范围选择流程，首次点击进入选区，拖拽选择起止帧后点击确认清空才执行。

接口契约：PropagationSeed 增加 propagation_seed_signature 字段，用于前端把未编辑传播结果绑定回原始 seed 传播链。

测试：补充前端 VideoWorkspace 范围清空、传播 lineage 传递测试；补充后端未编辑传播 seed 跳过重复传播、旧结果清理与换权重重传测试。

文档：同步更新 doc/03、doc/04、doc/07、doc/08、doc/09，明确 A/B 传播去重规则、清空片段范围选择和新增 seed signature 契约。

backend/tests/test_ai.py

@@ -2,7 +2,7 @@ import numpy as np
 import cv2
 from pathlib import Path
 from models import Annotation, ProcessingTask
-from services.propagation_task_runner import run_propagate_project_task
+from services.propagation_task_runner import _seed_signature, run_propagate_project_task
 
 
 def _create_project_and_frame(client):
@@ -614,6 +614,94 @@ def test_propagation_task_runner_replaces_legacy_or_different_weight_results(cli
     assert annotations[0].mask_data["polygons"] == [output_polygon]
 
 
+def test_propagation_task_runner_skips_unmodified_propagated_seed_on_overlapping_frames(client, db_session, monkeypatch):
+    project = client.post("/api/projects", json={"name": "Propagation Overlap Skip"}).json()
+    frames = [
+        client.post(f"/api/projects/{project['id']}/frames", json={
+            "project_id": project["id"],
+            "frame_index": idx,
+            "image_url": f"frames/{idx}.jpg",
+            "width": 640,
+            "height": 360,
+        }).json()
+        for idx in range(3)
+    ]
+
+    original_seed_polygon = [[0.1, 0.1], [0.2, 0.1], [0.2, 0.2]]
+    propagated_seed_polygon = [[0.14, 0.14], [0.24, 0.14], [0.24, 0.24]]
+    downstream_polygon = [[0.18, 0.18], [0.28, 0.18], [0.28, 0.28]]
+    inherited_signature = _seed_signature({
+        "polygons": [original_seed_polygon],
+        "label": "胆囊",
+        "color": "#ff0000",
+        "source_annotation_id": 7,
+        "source_mask_id": "annotation-7",
+    })
+
+    db_session.add(Annotation(
+        project_id=project["id"],
+        frame_id=frames[2]["id"],
+        mask_data={
+            "polygons": [downstream_polygon],
+            "label": "胆囊",
+            "color": "#ff0000",
+            "source": "sam2.1_hiera_tiny_propagation",
+            "propagated_from_frame_id": frames[0]["id"],
+            "propagation_seed_key": "annotation:7",
+            "propagation_seed_signature": inherited_signature,
+            "propagation_direction": "forward",
+            "source_annotation_id": 7,
+            "source_mask_id": "annotation-7",
+        },
+        bbox=[0.18, 0.18, 0.1, 0.1],
+    ))
+    db_session.commit()
+
+    task = ProcessingTask(
+        task_type="propagate_masks",
+        status="queued",
+        progress=0,
+        project_id=project["id"],
+        payload={
+            "project_id": project["id"],
+            "frame_id": frames[1]["id"],
+            "model": "sam2.1_hiera_tiny",
+            "include_source": False,
+            "save_annotations": True,
+            "steps": [{
+                "direction": "forward",
+                "max_frames": 2,
+                "seed": {
+                    "polygons": [propagated_seed_polygon],
+                    "label": "胆囊",
+                    "color": "#ff0000",
+                    "source_annotation_id": 7,
+                    "source_mask_id": "annotation-7",
+                    "propagation_seed_signature": inherited_signature,
+                },
+            }],
+        },
+    )
+    db_session.add(task)
+    db_session.commit()
+    db_session.refresh(task)
+
+    propagate_calls = []
+    monkeypatch.setattr("services.propagation_task_runner.download_file", lambda object_name: b"jpeg")
+    monkeypatch.setattr("services.propagation_task_runner.publish_task_progress_event", lambda event_task: None)
+    monkeypatch.setattr("services.propagation_task_runner.sam_registry.propagate_video", lambda *args, **kwargs: propagate_calls.append(args) or [])
+
+    result = run_propagate_project_task(db_session, task.id)
+
+    assert result["created_annotation_count"] == 0
+    assert result["deleted_annotation_count"] == 0
+    assert result["skipped_seed_count"] == 1
+    assert propagate_calls == []
+    annotations = db_session.query(Annotation).filter(Annotation.project_id == project["id"]).all()
+    assert len(annotations) == 1
+    assert annotations[0].mask_data["polygons"] == [downstream_polygon]
+
+
 def test_predict_validation_errors(client, monkeypatch):
     project, _, _ = _create_project_and_frame(client)