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/semantic-cover-fit.test.ts

@@ -0,0 +1,39 @@
+// @vitest-environment happy-dom
+
+import { describe, expect, it } from "vitest";
+import { constrainSemanticCoversToPage } from "../src/semantic-cover-fit.js";
+
+function createCover(height: number) {
+  const root = document.createElement("div");
+  root.innerHTML = `
+    <header
+      data-semantic-region="cover"
+      data-semantic-break-after="next-page"
+    >封面</header>
+  `;
+  const cover = root.querySelector<HTMLElement>("header")!;
+  cover.getBoundingClientRect = () =>
+    ({ height } as DOMRect);
+  return { root, cover };
+}
+
+describe("语义封面页面适配", () => {
+  it("只将超过横向页面内容区的封面收束为单页高度", () => {
+    const { root, cover } = createCover(900);
+
+    expect(constrainSemanticCoversToPage(root, 680)).toBe(1);
+    expect(cover.style.boxSizing).toBe("border-box");
+    expect(cover.style.height).toBe("680px");
+    expect(cover.style.minHeight).toBe("680px");
+    expect(cover.style.maxHeight).toBe("680px");
+    expect(cover.dataset.semanticCoverFit).toBe("constrained");
+  });
+
+  it("纵向页面可以容纳主题封面时保持主题原始高度", () => {
+    const { root, cover } = createCover(680);
+
+    expect(constrainSemanticCoversToPage(root, 900)).toBe(0);
+    expect(cover.getAttribute("style")).toBeNull();
+    expect(cover.dataset.semanticCoverFit).toBeUndefined();
+  });
+});