fix: 修复DOCX独立封面视觉门禁

统一 Chromium 与 DOCX 的受限高度封面布局，补齐可编辑边框、装饰边和 Office 页面舍入适配。

按四套独立封面主题、纵横方向及五组页边距执行 40 个 Word/WPS 真实视觉场景，严格失败为 0，并保留正文诊断供 D4 修复。

packages/preview-engine/tests/semantic-cover-fit.test.ts

@@ -14,19 +14,44 @@ function createCover(height: number) {
   const cover = root.querySelector<HTMLElement>("header")!;
   cover.getBoundingClientRect = () =>
     ({ height } as DOMRect);
+  document.body.append(root);
   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);
+    cover.style.display = "flex";
+    cover.style.flexDirection = "column";
+    cover.style.alignItems = "center";
+    cover.style.justifyContent = "center";
 
     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.style.height).toBe("672px");
+    expect(cover.style.minHeight).toBe("672px");
+    expect(cover.style.maxHeight).toBe("672px");
+    expect(cover.style.overflow).toBe("hidden");
+    expect(cover.style.display).toBe("block");
+    expect(cover.style.getPropertyValue("break-inside")).toBe("auto");
+    expect(cover.style.getPropertyPriority("break-inside")).toBe("important");
     expect(cover.dataset.semanticCoverFit).toBe("constrained");
+    expect(cover.dataset.semanticCoverPageBreak).toBe("natural");
+    expect(cover.hasAttribute("data-semantic-break-after")).toBe(false);
+    const anchor = cover.nextElementSibling as HTMLElement | null;
+    expect(anchor?.dataset.semanticCoverPageBreakAnchor).toBe("true");
+    expect(anchor?.style.height).toBe("0px");
+    expect(anchor?.style.getPropertyValue("break-after")).toBe("page");
+    expect(anchor?.style.getPropertyPriority("break-after")).toBe(
+      "important"
+    );
+    const layout = cover.querySelector<HTMLElement>(
+      '[data-semantic-cover-layout="flex"]'
+    );
+    expect(layout?.style.display).toBe("flex");
+    expect(layout?.style.flexDirection).toBe("column");
+    expect(layout?.style.alignItems).toBe("center");
+    expect(layout?.style.justifyContent).toBe("center");
   });
 
   it("纵向页面可以容纳主题封面时保持主题原始高度", () => {
@@ -36,4 +61,29 @@ describe("语义封面页面适配", () => {
     expect(cover.getAttribute("style")).toBeNull();
     expect(cover.dataset.semanticCoverFit).toBeUndefined();
   });
+
+  it("横向页面按同一比例压缩封面子元素纵向间距", () => {
+    const { root, cover } = createCover(900);
+    cover.style.display = "flex";
+    cover.style.flexDirection = "column";
+    cover.innerHTML = `
+      <p style="margin-top: 50px; margin-bottom: 50px">标题</p>
+      <p style="margin-top: 50px; margin-bottom: 50px">日期</p>
+    `;
+    for (const child of Array.from(cover.children)) {
+      Object.defineProperty(child, "getBoundingClientRect", {
+        value: () => ({ height: 100 } as DOMRect),
+      });
+    }
+
+    expect(constrainSemanticCoversToPage(root, 300)).toBe(1);
+    expect(cover.dataset.semanticCoverSpacingScale).toBe("0.460000");
+    const layout = cover.querySelector<HTMLElement>(
+      '[data-semantic-cover-layout="flex"]'
+    )!;
+    for (const child of Array.from(layout.children)) {
+      expect((child as HTMLElement).style.marginTop).toBe("23px");
+      expect((child as HTMLElement).style.marginBottom).toBe("23px");
+    }
+  });
 });