fix: correct table context offset calculation and dollar sign escaping in atomics

- Extract beforeStart variable to avoid redundant Math.max calls and fix
  offset calculation in expandTableContext
- Escape $ characters in atom content during restore to prevent special
  replacement patterns ($&, $1, etc.) from corrupting output

Author: Naraku215

lib/file/split-markdown/advanced/atomics.js

@@ -369,16 +369,18 @@ function expandTableContext(text, blocks, maxPrefixLen = 300, maxSuffixLen = 200
     // 向前查找前文段落
     if (block.start > 0) {
       // 找到表格前最近的两个 \n\n 之间的段落
-      const beforeText = text.substring(Math.max(0, block.start - maxPrefixLen), block.start);
+      const beforeStart = Math.max(0, block.start - maxPrefixLen);
+      const beforeText = text.substring(beforeStart, block.start);
       const lastParaBreak = beforeText.lastIndexOf('\n\n');
       if (lastParaBreak >= 0) {
         const paragraphText = beforeText.substring(lastParaBreak + 2).trim();
         if (paragraphText.length > 0 && paragraphText.length <= maxPrefixLen) {
-          newStart = Math.max(0, block.start - maxPrefixLen) + lastParaBreak + 2;
+          // lastParaBreak 是 beforeText 的相对偏移，需要加上 beforeStart 得到绝对位置
+          newStart = beforeStart + lastParaBreak + 2;
         }
       } else if (beforeText.trim().length > 0 && beforeText.trim().length <= maxPrefixLen) {
         // 没有段落分隔，整段前文都纳入
-        newStart = Math.max(0, block.start - maxPrefixLen);
+        newStart = beforeStart;
       }
     }
 
@@ -480,7 +482,9 @@ function restore(protectedText, atomMap) {
   let text = protectedText;
   for (let i = 0; i < atomMap.length; i++) {
     const placeholder = makePlaceholder(i);
-    text = text.replace(placeholder, atomMap[i].content);
+    // 转义替换字符串中的 $ 特殊字符（$$ → $, $& → 匹配等）
+    const escapedContent = atomMap[i].content.replace(/\$/g, '$$$$');
+    text = text.replace(placeholder, escapedContent);
   }
   return text;
 }