Tab inside a fenced code block in a block quote is expanded to spaces (regression in 18.1.2) #4184

This issue was posted by Claude Code using claude-opus-5-5 on behalf of David.

Marked version: 18.1.2. The code is unchanged at v18.1.3 and on master. 18.1.1 is not affected.

Describe the bug
A tab inside a fenced code block in a block quote is turned into spaces. CommonMark keeps it as a tab, because it is code content, not block structure.

#4110 added blockquoteMarkerWhitespace (/^( {0,3}>)([ \t]+)/gm). The blockquote tokenizer passes the whole whitespace run after > to expandTabs. That is right for example 6 (>\t\tfoo), where the tabs open an indented code block. In > \tname, though, the marker takes > and its optional space, and the tab is the first character of a fenced code line. Spec section 2.2 expands tabs only "in contexts where whitespace helps to define block structure", so this tab should stay a tab.

As #4110 notes, @markedjs/html-differ treats whitespace inside <pre> as equal, so the spec suite does not catch this.

To Reproduce

import { marked } from 'marked';

marked.parse('> ```python\n> class P:\n> \tname: str\n> ```\n');

marked 18.1.2:

<blockquote>
<pre><code class="language-python">class P:
  name: str
</code></pre>
</blockquote>

marked 18.1.1 and commonmark.js 0.31.2 (the dingus) both return:

<blockquote>
<pre><code class="language-python">class P:
	name: str
</code></pre>
</blockquote>

Expected behavior
The tab stays a tab, as in 18.1.1 and the CommonMark reference implementation. Only whitespace that the block quote marker or an indented code block consumes should be expanded.