添加取消选中实体按钮

- 在左侧工具栏拖拽/选择下方新增“取消选中”按钮，提供等同 Esc 的可点击入口。

- 将 VideoWorkspace 的取消选中信号传入 CanvasArea，统一清空 mask 选区、临时绘制状态和顶点选择。

- 修正 Canvas 本地选区与全局 selectedMaskIds 的同步，避免取消后旧本地选区被重新发布。

- 补充 ToolsPalette、CanvasArea 回归测试，覆盖实体按钮位置、回调和 clearSelectionSignal 行为。

- 更新 README、AGENTS 与前端审计/需求冻结/设计冻结/测试计划/交互状态机文档。

src/components/VideoWorkspace.tsx

@@ -503,6 +503,7 @@ export function VideoWorkspace({ onNavigateToAI }: { onNavigateToAI?: () => void
   const redoMasks = useStore((state) => state.redoMasks);
   const [isSaving, setIsSaving] = useState(false);
   const [isExporting, setIsExporting] = useState(false);
+  const [clearSelectionSignal, setClearSelectionSignal] = useState(0);
   const [isExportMenuOpen, setIsExportMenuOpen] = useState(false);
   const [exportScope, setExportScope] = useState<ExportScope>('current');
   const [exportOutputs, setExportOutputs] = useState<SegmentationExportOutput[]>([
@@ -924,6 +925,11 @@ export function VideoWorkspace({ onNavigateToAI }: { onNavigateToAI?: () => void
     });
   }, [currentFrame, currentFrameNumber, executeClearCurrentMasks]);
 
+  const handleClearSelection = useCallback(() => {
+    setSelectedMaskIds([]);
+    setClearSelectionSignal((value) => value + 1);
+  }, [setSelectedMaskIds]);
+
   const handleDeleteSelectedMasks = useCallback(async (requestedMaskIds?: string[]) => {
     if (!currentFrame) return;
     const latestMasks = useStore.getState().masks;
@@ -2269,6 +2275,7 @@ export function VideoWorkspace({ onNavigateToAI }: { onNavigateToAI?: () => void
           onTriggerAI={onNavigateToAI}
           onAutoPropagate={() => void handleAutoPropagate()}
           onImportGtMask={() => gtMaskInputRef.current?.click()}
+          onClearSelection={handleClearSelection}
           onDeleteMasks={handleDeleteSelectedMasks}
           onClearMasks={handleClearCurrentFrameMasks}
           canAutoPropagate={Boolean(currentProject?.id && currentFrame?.id) && !isSaving && !isExporting && !isImportingGt}
@@ -2284,6 +2291,7 @@ export function VideoWorkspace({ onNavigateToAI }: { onNavigateToAI?: () => void
               frame={currentFrame}
               currentFrameNumber={currentFrameNumber || 0}
               totalFrames={totalFrames}
+              clearSelectionSignal={clearSelectionSignal}
               onRequestDeleteMasks={(maskIds) => void handleDeleteSelectedMasks(maskIds)}
               onRequestBooleanFrameRange={handleBooleanFrameRangeRequest}
               onBooleanOperationStart={clearPendingBooleanRangeSelection}