feat: 完成 DOCX R4 元素级视觉门禁

建立 Chromium、Word 与 WPS 的可编辑内容、段落几何、页眉页脚、封面、媒体和逐页视觉比较链路。

支持封面独立分节、正文页码重启、主题页边距与横向纸张，并将自然分页差异降为诊断项。

修正代码块内边距和折叠表格单元格边框映射，14 套主题生产矩阵与六组布局视觉矩阵通过。

packages/preview-engine/tests/paged-preview.test.ts

@@ -26,6 +26,14 @@ const payload: PagedPreviewPayload = {
     keywords: [],
     language: "zh-CN"
   },
+  semanticDocument: {
+    schemaVersion: 1,
+    titlePolicy: {
+      metadataTitle: "suppress",
+      firstBodyHeading: "keep"
+    },
+    regions: []
+  },
   features: [],
   themeCss: "#write { color: #333; }",
   exportConfig: defaultExportConfig
@@ -94,6 +102,9 @@ describe("分页预览协议", () => {
     );
     expect(css).toContain('content: "内网团队"');
     expect(css).toContain("border-bottom: 0.2mm solid currentColor");
+    expect(css).toContain(
+      "transform: translateY(calc(16mm - 8mm - 3mm))"
+    );
   });
 
   it("支持自定义页码模板和起始页码", () => {
@@ -192,6 +203,40 @@ describe("分页预览协议", () => {
     );
   });
 
+  it("由语义文档强制封面结束后换页", () => {
+    expect(documentGeometryCss).toContain(
+      '#write [data-semantic-break-after="next-page"]'
+    );
+    expect(documentGeometryCss).toContain(
+      "break-after: page !important"
+    );
+    expect(documentGeometryCss).toContain(
+      "page-break-after: always !important"
+    );
+    expect(documentGeometryCss).toContain(
+      '[data-semantic-region="cover"]'
+    );
+    expect(documentGeometryCss).toContain(
+      "break-inside: avoid !important"
+    );
+  });
+
+  it("使用标准 Letter 横向尺寸生成分页规则", () => {
+    const css = buildPagedMediaCss(
+      {
+        ...defaultExportConfig,
+        paper: {
+          ...defaultExportConfig.paper,
+          format: "Letter",
+          orientation: "landscape"
+        }
+      },
+      payload
+    );
+
+    expect(css).toContain("size: 279.4mm 215.9mm;");
+  });
+
   it("避免 Typora 围栏容器重复应用行内代码盒模型", () => {
     expect(documentBaseCss).toContain(
       "#write pre.md-fences > code"