GHSA-5vp3-3cg6-2rq3

## Summary

`to_markdown()` is vulnerable when serializing attacker-controlled `<pre>` content. The `<pre>` handler emits a fixed three-backtick fenced code block, but writes decoded text content into that fence without choosing a delimiter longer than any backtick run inside the content.

An attacker can place backticks and HTML-like text inside a sanitized `<pre>` element so that the generated Markdown closes the fence early and leaves raw HTML outside the code block. When that Markdown is rendered by a CommonMark/GFM-style renderer that allows raw HTML, the HTML executes.

This is a bypass of the v1.12.0 Markdown hardening. That fix escaped HTML-significant characters for regular text nodes, but `<pre>` uses a separate serialization path and does not apply the same protection.

## Details

The vulnerable `<pre>` Markdown path:

- extracts decoded text from the `<pre>` subtree - opens a fenced block with a fixed delimiter of `````` - writes the decoded text directly into the output - closes with another fixed ``````

Because the fence length is fixed, attacker-controlled content containing a backtick run of length 3 or more can terminate the code block. If the content also contains decoded HTML-like text such as `&lt;img ...&gt;`, that text appears outside the fence in the resulting Markdown and is treated as raw HTML by downstream Markdown renderers.

The issue is not that HTML-like text appears inside code blocks. The issue is that the serializer allows attacker-controlled `<pre>` text to break out of the fixed fence.

## Reproduction

```python from justhtml import JustHTML

payload = "<pre>&#96;&#96;&#96;\n&lt;img src=x onerror=alert(1)&gt;</pre>" doc = JustHTML(payload, fragment=True) # default sanitize=True

print(doc.to_html(pretty=False)) # <pre>``` # &lt;img src=x onerror=alert(1)&gt;</pre>

print(doc.to_markdown()) # ``` # ``` # <img src=x onerror=alert(1)> # ```

```

Rendered as CommonMark/GFM-style Markdown, that output is interpreted as:

1. Line 1 opens a fenced code block 2. Line 2 closes it 3. Line 3 is raw HTML outside the fence 4. Line 4 opens a new fence

## Impact

Applications that treat `JustHTML(..., sanitize=True).to_markdown()` output as safe for direct rendering in Markdown contexts may be exposed to XSS, depending on the downstream Markdown renderer's raw-HTML handling.

## Root Cause

The `<pre>` Markdown serializer uses a fixed fence instead of selecting a delimiter longer than the longest backtick run in the content.

## Fix

When serializing `<pre>` content to Markdown, choose a fence length longer than any backtick run present in the code block content, with a minimum length of 3.