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/src/docx-media-runtime.ts

@@ -5,6 +5,7 @@ import {
   MAXIMUM_DOCX_RESOURCE_COUNT,
   type DocxMediaCapturePlan,
   type DocxMediaCaptureTarget,
+  type DocxMediaAlignment,
   type DocxMediaKind,
   type PagedDocumentPayload
 } from "@md-to-pdf/core";
@@ -93,6 +94,33 @@ function fitElementToContent(
   return element.getBoundingClientRect();
 }
 
+function getMediaAlignment(
+  article: HTMLElement,
+  rect: DOMRect
+): DocxMediaAlignment {
+  const articleRect = article.getBoundingClientRect();
+  const articleWidth = finitePositive(articleRect.width, rect.width);
+  if (rect.width >= articleWidth - 2) {
+    return "center";
+  }
+
+  const leftScore = Math.abs(rect.left - articleRect.left);
+  const centerScore = Math.abs(
+    (rect.left + rect.right) / 2 -
+      (articleRect.left + articleRect.right) / 2
+  );
+  const rightScore = Math.abs(articleRect.right - rect.right);
+  const minimumScore = Math.min(
+    leftScore,
+    centerScore,
+    rightScore
+  );
+  if (centerScore <= minimumScore + 2) {
+    return "center";
+  }
+  return leftScore <= rightScore ? "left" : "right";
+}
+
 function getRasterScale(width: number, height: number) {
   return Math.min(
     DOCX_MEDIA_RASTER_SCALE,
@@ -179,6 +207,7 @@ export function collectDocxMediaCaptureTargets(
       ...(getCaption(element)
         ? { caption: getCaption(element) }
         : {}),
+      alignment: getMediaAlignment(article, rect),
       displayWidthPx: width,
       displayHeightPx: height,
       captureX,