Removed ~60 lines of inline frontmatter parsing (FMState, Frontmatter,
ParseResult types, fmFoldLine, parseFrontmatter, and accessor functions).
Now uses the frontmatter package via import.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Remove 606 lines of hand-rolled markdown parsing from main.lux and the
unused markdown.lux split file. Replace with `import markdown` using the
new markdown package (path dependency at ../markdown).
This fixes the heading-in-list rendering bug where `- ### Title` was
showing literal `### ` text. Now renders as `<li><h3>Title</h3></li>`.
Also adds strikethrough support (~~text~~).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
