feat: 完善 AI 分割与工作区标注闭环

功能增加:

- 将视频导入和生成帧拆成两个明确动作，项目库生成帧时选择 FPS，工作区不再自动触发拆帧。

- 为工作区新增调整多边形工具，支持选中 mask、拖动顶点、边中点插点、双击边界按位置插点，并保留多 polygon 子区域编辑。

- 打通 AI 页 SAM2/SAM3 结果到工作区的联动，生成 mask 后自动选中，可在右侧分类树换标签，并推送到工作区继续编辑。

- 增强 Dashboard WebSocket 连接状态与心跳，使用真实 onopen/onclose/onerror 状态驱动前端显示。

- 完善 SAM3 external worker 适配，支持 box prompt、semantic 请求级阈值和 video tracker 路径。

bugfix:

- 修复 SAM2 文本语义误走自动分割的问题，改为提示使用点提示或切换 SAM3。

- 修复 SAM2 多候选重叠显示的问题，点提示和 auto fallback 默认只采用最高分候选。

- 修复 SAM2 反向点看起来无效的问题，带负点时启用背景过滤，过滤为空时移除旧候选。

- 修复 SAM3 单个 2D mask 结果无法转 polygon、低阈值 semantic 返回被默认阈值吞掉的问题。

- 修复 AI 页 mask 未选中导致分类树无法修改 SAM2 结果标签的问题。

测试和文档:

- 补充 CanvasArea、AISegmentation、ProjectLibrary、VideoWorkspace、Dashboard、websocket 和 SAM engine/API 测试。

- 新增 backend/tests/test_sam2_engine.py，覆盖 SAM2 单候选请求和 auto fallback 行为。

- 更新 README、AGENTS 和 doc 需求/设计/接口/测试矩阵，按当前实现冻结功能状态。

src/components/CanvasArea.tsx

@@ -19,6 +19,7 @@ type PromptBox = { x1: number; y1: number; x2: number; y2: number };
 
 const DRAG_MANUAL_TOOLS = new Set(['create_rectangle', 'create_circle', 'create_line']);
 const POLYGON_TOOL = 'create_polygon';
+const EDIT_POLYGON_TOOL = 'edit_polygon';
 const POINT_TOOL = 'create_point';
 const BOOLEAN_TOOLS = new Set(['area_merge', 'area_remove']);
 const POLYGON_CLOSE_RADIUS = 8;
@@ -95,6 +96,32 @@ function pointDistance(a: CanvasPoint, b: CanvasPoint): number {
   return Math.hypot(a.x - b.x, a.y - b.y);
 }
 
+function distanceToSegmentSquared(point: CanvasPoint, start: CanvasPoint, end: CanvasPoint): number {
+  const dx = end.x - start.x;
+  const dy = end.y - start.y;
+  const lengthSquared = dx * dx + dy * dy;
+  if (lengthSquared === 0) {
+    return (point.x - start.x) ** 2 + (point.y - start.y) ** 2;
+  }
+  const t = clamp(((point.x - start.x) * dx + (point.y - start.y) * dy) / lengthSquared, 0, 1);
+  const projected = { x: start.x + t * dx, y: start.y + t * dy };
+  return (point.x - projected.x) ** 2 + (point.y - projected.y) ** 2;
+}
+
+function nearestPolygonEdgeIndex(points: CanvasPoint[], point: CanvasPoint): number {
+  return points.reduce((bestIndex, start, index) => {
+    const end = points[(index + 1) % points.length];
+    if (!end) return bestIndex;
+    const bestStart = points[bestIndex];
+    const bestEnd = points[(bestIndex + 1) % points.length];
+    const currentDistance = distanceToSegmentSquared(point, start, end);
+    const bestDistance = bestStart && bestEnd
+      ? distanceToSegmentSquared(point, bestStart, bestEnd)
+      : Number.POSITIVE_INFINITY;
+    return currentDistance < bestDistance ? index : bestIndex;
+  }, 0);
+}
+
 function segmentationArea(segmentation?: number[][]): number {
   return (segmentation || []).reduce((sum, polygon) => sum + polygonArea(flatPolygonToPoints(polygon)), 0);
 }
@@ -210,10 +237,11 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
   const [manualStart, setManualStart] = useState<CanvasPoint | null>(null);
   const [manualCurrent, setManualCurrent] = useState<CanvasPoint | null>(null);
   const [polygonPoints, setPolygonPoints] = useState<CanvasPoint[]>([]);
-  const [selectedMaskId, setSelectedMaskId] = useState<string | null>(null);
-  const [selectedMaskIds, setSelectedMaskIds] = useState<string[]>([]);
+  const [selectedMaskId, setSelectedMaskId] = useState<string | null>(() => useStore.getState().selectedMaskIds[0] || null);
+  const [selectedMaskIds, setSelectedMaskIds] = useState<string[]>(() => useStore.getState().selectedMaskIds);
   const [selectedPolygonIndex, setSelectedPolygonIndex] = useState(0);
   const [selectedVertexIndex, setSelectedVertexIndex] = useState<number | null>(null);
+  const previousFrameIdRef = useRef<string | undefined>(frame?.id);
   const [isInferencing, setIsInferencing] = useState(false);
   const [inferenceMessage, setInferenceMessage] = useState('');
 
@@ -253,6 +281,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
   const draftMaskCount = frameMasks.filter((mask) => !mask.annotationId).length;
   const dirtyMaskCount = frameMasks.filter((mask) => mask.saveStatus === 'dirty').length;
   const isBooleanTool = BOOLEAN_TOOLS.has(effectiveTool);
+  const isPolygonEditTool = effectiveTool === 'move' || effectiveTool === EDIT_POLYGON_TOOL;
 
   useEffect(() => {
     const handleResize = () => {
@@ -273,11 +302,22 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
     setManualStart(null);
     setManualCurrent(null);
     setPolygonPoints([]);
+    setSelectedVertexIndex(null);
+    if (!isPolygonEditTool && !isBooleanTool) {
+      setSelectedMaskId(null);
+      setSelectedMaskIds([]);
+      setSelectedPolygonIndex(0);
+    }
+  }, [effectiveTool, isBooleanTool, isPolygonEditTool]);
+
+  useEffect(() => {
+    if (previousFrameIdRef.current === frame?.id) return;
+    previousFrameIdRef.current = frame?.id;
     setSelectedMaskId(null);
     setSelectedMaskIds([]);
     setSelectedPolygonIndex(0);
     setSelectedVertexIndex(null);
-  }, [effectiveTool, frame?.id]);
+  }, [frame?.id]);
 
   useEffect(() => {
     setPoints([]);
@@ -420,6 +460,10 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
     setIsInferencing(true);
     setInferenceMessage('');
     try {
+      const hasNegativePrompt = Boolean(promptPoints?.some((point) => point.type === 'neg'));
+      const existingCandidate = !options.resetCandidate && samCandidateMaskId
+        ? masks.find((mask) => mask.id === samCandidateMaskId)
+        : null;
       const result = await predictMask({
         imageId: frame.id,
         imageWidth,
@@ -429,13 +473,11 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
           ? promptPoints.map((p) => ({ x: p.x, y: p.y, type: p.type }))
           : undefined,
         box: promptBox,
+        ...(hasNegativePrompt ? { options: { auto_filter_background: true, min_score: 0.05 } } : {}),
       });
 
       const [m] = result.masks;
       if (m) {
-        const existingCandidate = !options.resetCandidate && samCandidateMaskId
-          ? masks.find((mask) => mask.id === samCandidateMaskId)
-          : null;
         const label = activeClass?.name || existingCandidate?.label || m.label;
         const color = activeClass?.color || existingCandidate?.color || m.color;
         const metadata = {
@@ -443,6 +485,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
           source: aiModel === 'sam3' ? 'sam3_box' : 'sam2_interactive',
           promptBox: promptBox || null,
           promptPointCount: promptPoints?.length || 0,
+          promptNegativePointCount: promptPoints?.filter((point) => point.type === 'neg').length || 0,
         };
         const nextMask = {
           frameId: frame.id,
@@ -476,7 +519,15 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
           });
         }
       } else {
-        setInferenceMessage('模型没有返回可用区域，请调整点/框提示后重试。');
+        if (existingCandidate && hasNegativePrompt) {
+          setMasks(masks.filter((mask) => mask.id !== existingCandidate.id));
+          setSamCandidateMaskId(null);
+          setSelectedMaskId(null);
+          setSelectedMaskIds([]);
+          setInferenceMessage('反向点已排除当前候选区域，请重新框选或添加新的正向点。');
+        } else {
+          setInferenceMessage('模型没有返回可用区域，请调整点/框提示后重试。');
+        }
       }
     } catch (err) {
       console.error('Inference failed:', err);
@@ -485,7 +536,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
     } finally {
       setIsInferencing(false);
     }
-  }, [activeClass, activeTemplateId, addMask, aiModel, frame?.height, frame?.id, frame?.width, image?.height, image?.naturalHeight, image?.naturalWidth, image?.width, masks, samCandidateMaskId, updateMask]);
+  }, [activeClass, activeTemplateId, addMask, aiModel, frame?.height, frame?.id, frame?.width, image?.height, image?.naturalHeight, image?.naturalWidth, image?.width, masks, samCandidateMaskId, setMasks, updateMask]);
 
   const handleApplyActiveClass = () => {
     if (!frame?.id || !activeClass) return;
@@ -598,7 +649,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
   };
 
   const handleStageClick = (e: any) => {
-    if (effectiveTool === 'move') return;
+    if (isPolygonEditTool) return;
     if (effectiveTool === 'box_select') return; // handled by mouseup
     if (DRAG_MANUAL_TOOLS.has(effectiveTool)) return;
 
@@ -716,7 +767,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
 
     window.addEventListener('keydown', handleKeyDown);
     return () => window.removeEventListener('keydown', handleKeyDown);
-  }, [deleteMasksById, effectiveTool, finishPolygon, polygonPoints, redoMasks, selectedMask, selectedMaskIds, selectedPolygonIndex, selectedVertexIndex, undoMasks, updatePolygonMask]);
+  }, [deleteMasksById, effectiveTool, finishPolygon, isPolygonEditTool, polygonPoints, redoMasks, selectedMask, selectedMaskIds, selectedPolygonIndex, selectedVertexIndex, undoMasks, updatePolygonMask]);
 
   const boxRect = React.useMemo(() => {
     if (!boxStart || !boxCurrent) return null;
@@ -753,7 +804,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
   };
 
   const handleMaskSelect = (mask: Mask, event: any, polygonIndex = 0) => {
-    if (effectiveTool !== 'move' && !isBooleanTool) return;
+    if (!isPolygonEditTool && !isBooleanTool) return;
     event.cancelBubble = true;
     if (isBooleanTool) {
       setSelectedMaskIds((current) => (
@@ -807,6 +858,25 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
     updatePolygonMask(mask, nextPoints, selectedPolygonIndex);
   };
 
+  const handlePathDoubleClick = (mask: Mask, event: any, polygonIndex = 0) => {
+    if (effectiveTool !== EDIT_POLYGON_TOOL) return;
+    event.cancelBubble = true;
+    const point = stagePoint(event);
+    const currentPoints = segmentationToPoints(mask.segmentation, polygonIndex);
+    if (!point || currentPoints.length < 3) return;
+    const edgeIndex = nearestPolygonEdgeIndex(currentPoints, point);
+    const nextPoints = [
+      ...currentPoints.slice(0, edgeIndex + 1),
+      point,
+      ...currentPoints.slice(edgeIndex + 1),
+    ];
+    setSelectedMaskId(mask.id);
+    setSelectedMaskIds([mask.id]);
+    setSelectedPolygonIndex(polygonIndex);
+    setSelectedVertexIndex(edgeIndex + 1);
+    updatePolygonMask(mask, nextPoints, polygonIndex);
+  };
+
   const handleBooleanOperation = async () => {
     if (!frame || booleanSelectedMasks.length < 2) return;
     const primary = booleanSelectedMasks[0];
@@ -918,6 +988,8 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
                       strokeWidth={(selectedMaskIds.includes(mask.id) ? 2 : 1) / scale}
                       onClick={(event: any) => handleMaskSelect(mask, event, polygonIndex)}
                       onTap={(event: any) => handleMaskSelect(mask, event, polygonIndex)}
+                      onDblClick={(event: any) => handlePathDoubleClick(mask, event, polygonIndex)}
+                      onDblTap={(event: any) => handlePathDoubleClick(mask, event, polygonIndex)}
                     />
                   ))}
                 </Group>
@@ -987,7 +1059,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
             )))}
 
             {/* Polygon edge insertion handles */}
-            {!isBooleanTool && selectedMask && selectedMaskPoints.map((point, index) => {
+            {isPolygonEditTool && selectedMask && selectedMaskPoints.map((point, index) => {
               const next = selectedMaskPoints[(index + 1) % selectedMaskPoints.length];
               if (!next) return null;
               return (
@@ -1006,7 +1078,7 @@ export function CanvasArea({ activeTool, frame, onClearMasks, onDeleteMaskAnnota
             })}
 
             {/* Polygon vertex editor */}
-            {!isBooleanTool && selectedMask && selectedMaskPoints.map((point, index) => (
+            {isPolygonEditTool && selectedMask && selectedMaskPoints.map((point, index) => (
               <Circle
                 key={`${selectedMask.id}-vertex-${selectedPolygonIndex}-${index}`}
                 x={point.x}