Wrap content_html in the Hematite dark-theme shell.
content_html is everything inside .wrap — assemble header cards,
sections, etc. in the caller. The shell provides CSS, JS, and the footer.
Shared dark-theme HTML shell for all Hematite HTML outputs.
Any feature that saves an HTML file calls build_html_shell — consistent
look everywhere, CSS and JS defined in one place.
Convert a subset of Markdown to HTML suitable for research/note pages.
Handles: ATX headings, fenced code blocks, inline code, bold, bullet lists,
blank-line-separated paragraphs, and bare URLs as links.