Function write_file 

pub fn write_file(
    path: &Path,
    frontmatter: &Frontmatter,
    body: &str,
) -> Result<(), ParseError>

Atomically write a markdown file from frontmatter + body: emit the frontmatter in canonical key order, then the body verbatim, via a temp-file-rename so a reader never sees a half-written file. Preserves the operator-edited body exactly as given.