取消画笔自动合并选中遮罩

- 移除画笔与选中 mask 连通或重叠时自动 union 到旧 mask 的逻辑，画笔每次松手都生成新的独立 mask。

- 更新画笔工具提示，明确画笔会生成新的独立 mask。

- 调整 CanvasArea 回归测试，验证画笔触碰选中 mask 时仍保留旧 mask 并新建独立 mask。

- 更新 README、AGENTS、前端审计、需求冻结和测试计划文档，移除画笔自动合并描述。

src/components/CanvasArea.test.tsx

@@ -1723,10 +1723,10 @@ describe('CanvasArea', () => {
     }
   });
 
-  it('merges a connected brush stroke into the selected mask', () => {
+  it('creates an independent brush mask even when it touches the selected mask', () => {
     useStore.setState({
       activeTemplateId: '2',
-      activeClass: { id: 'c1', name: '胆囊', color: '#ff0000', zIndex: 20 },
+      activeClass: { id: 'c1', name: '胆囊', color: '#ff0000', zIndex: 20, maskId: 1 },
       activeClassId: 'c1',
       selectedMaskIds: ['m1'],
       masks: [
@@ -1749,15 +1749,23 @@ describe('CanvasArea', () => {
     fireEvent.mouseMove(stage, { clientX: 170, clientY: 100 });
     fireEvent.mouseUp(stage, { clientX: 210, clientY: 110 });
 
-    expect(useStore.getState().masks).toHaveLength(1);
+    expect(useStore.getState().masks).toHaveLength(2);
     expect(useStore.getState().masks[0]).toEqual(expect.objectContaining({
       id: 'm1',
       label: '胆囊',
       color: '#ff0000',
-      saveStatus: 'draft',
+      area: 2500,
     }));
-    expect(useStore.getState().masks[0].area).toBeGreaterThan(2500);
-    expect(useStore.getState().selectedMaskIds).toEqual(['m1']);
+    expect(useStore.getState().masks[1]).toEqual(expect.objectContaining({
+      label: '胆囊',
+      color: '#ff0000',
+      classId: 'c1',
+      classMaskId: 1,
+      saveStatus: 'draft',
+      metadata: expect.objectContaining({ shape: '画笔' }),
+    }));
+    expect(useStore.getState().masks[1].area).toBeGreaterThan(1000);
+    expect(useStore.getState().selectedMaskIds).toEqual([useStore.getState().masks[1].id]);
   });
 
   it('erases from the selected mask with a sampled stroke', () => {

src/components/CanvasArea.tsx

@@ -512,10 +512,6 @@ function imageBoundsGeometry(width: number, height: number): MultiPolygon | null
   ]]);
 }
 
-function geometriesOverlap(first: MultiPolygon, second: MultiPolygon): boolean {
-  return polygonClipping.intersection(first, second).length > 0;
-}
-
 export function CanvasArea({
   activeTool,
   frame,
@@ -643,7 +639,7 @@ export function CanvasArea({
       return {
         title: '画笔',
         body: activeClass
-          ? '按住并拖动画出连续区域；若与当前选中 mask 连通，会自动合并到该 mask。'
+          ? '按住并拖动画出连续区域，松开后生成一个新的独立 mask。'
           : '先在右侧语义分类树选择类别，然后按住并拖动画出连续区域。',
       };
     }
@@ -1150,34 +1146,6 @@ export function CanvasArea({
         return;
       }
 
-      const targetGeometry = selectedMask ? maskToMultiPolygon(selectedMask) : null;
-      const shouldMerge = Boolean(targetGeometry && geometriesOverlap(targetGeometry, strokeGeometry));
-      if (selectedMask && targetGeometry && shouldMerge) {
-        const resultGeometry = polygonClipping.union(targetGeometry, strokeGeometry);
-        const resultSegmentation = multiPolygonToSegmentation(resultGeometry);
-        if (resultSegmentation.length === 0) return;
-        const nextMask = {
-          ...maskWithSegmentation(selectedMask, resultSegmentation, {
-            area: multiPolygonArea(resultGeometry),
-            hasHoles: multiPolygonHasHoles(resultGeometry),
-            polygonRingCounts: multiPolygonRingCounts(resultGeometry),
-          }),
-          templateId: activeTemplateId || selectedMask.templateId,
-          classId: activeClass.id,
-          className: activeClass.name,
-          classZIndex: activeClass.zIndex,
-          classMaskId: activeClass.maskId,
-          label: activeClass.name,
-          color: activeClass.color,
-        };
-        setMasks(masks.map((mask) => (mask.id === selectedMask.id ? nextMask : mask)));
-        setSelectedMaskId(selectedMask.id);
-        setSelectedMaskIds([selectedMask.id]);
-        setGlobalSelectedMaskIds([selectedMask.id]);
-        setSelectedVertexIndex(null);
-        return;
-      }
-
       const nextMask = createManualMaskFromGeometry('画笔', strokeGeometry);
       if (nextMask) {
         setSelectedMaskId(nextMask.id);