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/src/paged-document-runtime.ts

@@ -898,6 +898,7 @@ export class PagedDocumentRuntime {
 
     const finalizeStartedAt = performance.now();
     const pageSequence = classifyPagedPages(this.root, payload);
+    pageCount = pageSequence.physicalPageCount;
     applyPagedPageDecorations(this.root, payload, pageSequence);
     if (options.target === "pdf") {
       removeTrailingPdfPageBreak(this.root);

packages/preview-engine/src/paged-preview.ts

@@ -167,11 +167,6 @@ body,
   page-break-after: always !important;
 }
 
-#write [data-semantic-region="cover"][data-semantic-break-after="next-page"] {
-  break-inside: avoid !important;
-  page-break-inside: avoid !important;
-}
-
 #write {
   width: 100% !important;
   max-width: none !important;

packages/preview-engine/src/semantic-cover-fit.ts

@@ -1,4 +1,105 @@
 const COVER_HEIGHT_EPSILON_PX = 0.5;
+const COVER_PAGE_BREAK_SAFETY_PX = 8;
+
+function pixelValue(value: string) {
+  const parsed = Number.parseFloat(value);
+  return Number.isFinite(parsed) ? parsed : 0;
+}
+
+function constrainCoverChildSpacing(
+  cover: HTMLElement,
+  pageContentHeightPx: number
+) {
+  const coverStyle = getComputedStyle(cover);
+  const availableHeightPx = Math.max(
+    0,
+    pageContentHeightPx -
+      pixelValue(coverStyle.paddingTop) -
+      pixelValue(coverStyle.paddingBottom) -
+      pixelValue(coverStyle.borderTopWidth) -
+      pixelValue(coverStyle.borderBottomWidth)
+  );
+  const children = Array.from(cover.children).flatMap((child) => {
+    if (!(child instanceof HTMLElement)) {
+      return [];
+    }
+    const style = getComputedStyle(child);
+    if (style.display === "none" || style.position === "absolute") {
+      return [];
+    }
+    return [{
+      element: child,
+      heightPx: child.getBoundingClientRect().height,
+      marginTopPx: pixelValue(style.marginTop),
+      marginBottomPx: pixelValue(style.marginBottom)
+    }];
+  });
+  const fixedHeightPx = children.reduce(
+    (total, child) => total + child.heightPx,
+    0
+  );
+  const spacingHeightPx = children.reduce(
+    (total, child) =>
+      total + child.marginTopPx + child.marginBottomPx,
+    0
+  );
+  if (
+    children.length === 0 ||
+    spacingHeightPx <= COVER_HEIGHT_EPSILON_PX ||
+    fixedHeightPx + spacingHeightPx <=
+      availableHeightPx + COVER_HEIGHT_EPSILON_PX
+  ) {
+    return 1;
+  }
+  const spacingScale = Math.min(
+    1,
+    Math.max(0, availableHeightPx - fixedHeightPx) / spacingHeightPx
+  );
+  for (const child of children) {
+    child.element.style.marginTop =
+      `${child.marginTopPx * spacingScale}px`;
+    child.element.style.marginBottom =
+      `${child.marginBottomPx * spacingScale}px`;
+  }
+  cover.dataset.semanticCoverSpacingScale =
+    spacingScale.toFixed(6);
+  return spacingScale;
+}
+
+function stabilizeConstrainedCoverFlow(cover: HTMLElement) {
+  const style = getComputedStyle(cover);
+  if (style.display !== "flex" && style.display !== "inline-flex") {
+    return;
+  }
+  const layout = cover.ownerDocument.createElement("div");
+  layout.dataset.semanticCoverLayout = "flex";
+  layout.style.display = "flex";
+  layout.style.width = "100%";
+  layout.style.height = "100%";
+  layout.style.minHeight = "0";
+  layout.style.flexDirection = style.flexDirection;
+  layout.style.alignItems = style.alignItems;
+  layout.style.justifyContent = style.justifyContent;
+  layout.append(...Array.from(cover.childNodes));
+  cover.append(layout);
+  cover.style.display = "block";
+}
+
+function insertCoverPageBreakAnchor(cover: HTMLElement) {
+  const anchor = cover.ownerDocument.createElement("div");
+  anchor.dataset.semanticCoverPageBreakAnchor = "true";
+  anchor.setAttribute("aria-hidden", "true");
+  anchor.style.cssText = [
+    "height: 0",
+    "margin: 0",
+    "padding: 0",
+    "border: 0",
+    "line-height: 0"
+  ].join(";");
+  anchor.style.setProperty("break-after", "page", "important");
+  anchor.style.setProperty("page-break-after", "always", "important");
+  cover.after(anchor);
+}
 
 export const semanticCoverSelector =
   '[data-semantic-region="cover"][data-semantic-break-after="next-page"]';
@@ -21,12 +122,24 @@ export function constrainSemanticCoversToPage(
     ) {
       continue;
     }
-    const constrainedHeight = `${pageContentHeightPx}px`;
+    const fittedHeightPx = Math.max(
+      1,
+      pageContentHeightPx - COVER_PAGE_BREAK_SAFETY_PX
+    );
+    const constrainedHeight = `${fittedHeightPx}px`;
     cover.style.boxSizing = "border-box";
     cover.style.height = constrainedHeight;
     cover.style.minHeight = constrainedHeight;
     cover.style.maxHeight = constrainedHeight;
+    cover.style.overflow = "hidden";
+    cover.style.setProperty("break-inside", "auto", "important");
+    cover.style.setProperty("page-break-inside", "auto", "important");
     cover.dataset.semanticCoverFit = "constrained";
+    cover.dataset.semanticCoverPageBreak = "natural";
+    cover.removeAttribute("data-semantic-break-after");
+    constrainCoverChildSpacing(cover, fittedHeightPx);
+    stabilizeConstrainedCoverFlow(cover);
+    insertCoverPageBreakAnchor(cover);
     constrainedCount += 1;
   }
   return constrainedCount;