Function render_to_html 

pub fn render_to_html(src: &str, theme: Theme, dense: bool) -> RenderResult

Render a markdown source string to a complete HTML document.

Pipeline:

1. Extract frontmatter (title, subtitle, mode, toc)
2. Parse body into comrak AST
3. Walk AST — transform directive HTML comments into layout HTML
4. Render AST to HTML with syntax highlighting
5. Wrap in themed CSS + header