`##` in markdown comments are not escaped but produce new header in rendered output

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

When using comments that contains markdown heading like

<!--
## Some Title
-->

the generated pdf contains a chapter called "Some Title"

Steps to reproduce

1. quarto create my-book-name (choose book type, not tested for other types)
2. Add the following lines to eg the first lines of index.qmd:

<!--
## Some Title
-->

3. quarto preview --to pdf -> Observe that Some Title is a separate chapter (although it is commented out)

Actual behavior

Some Title is a separate chapter (although it is commented out)

Expected behavior

Some Title - section doesn't render

Your environment

• Fedora42

Quarto check output

Quarto 1.8.25
[✓] Checking environment information...
Quarto cache location: /home/alex/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
Pandoc version 3.6.3: OK
Dart Sass version 1.87.0: OK
Deno version 2.3.1: OK
Typst version 0.13.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
Version: 1.8.25
Path: /opt/quarto-1.8.25/bin

[✓] Checking tools....................OK
Chromium: 869685
TinyTeX: (not installed)

[✓] Checking LaTeX....................OK
Using: Installation From Path
Path: /usr/bin
Version: 2023

[✓] Checking Chrome Headless....................OK
Using: Chrome found on system
Path: /usr/bin/chromium-browser
Source: PATH

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
Version: 3.13.9
Path: /usr/bin/python3
Jupyter: 5.7.2
Kernels: .venv, myenv, python3, rust

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
Version: 4.5.2
Path: /usr/lib64/R
LibPaths:
- /home/alex/R/x86_64-redhat-linux-gnu-library/4.5
- /usr/lib64/R/library
- /usr/share/R/library
knitr: 1.45
rmarkdown: 2.24

[✓] Checking Knitr engine render......OK

Reproduce the issue with quarto create my-book-name, the index.qmd comment, and quarto preview --to pdf. Trace how the commented heading reaches PDF rendering; done means ## Some Title inside the HTML comment no longer creates a separate chapter.