When a document begins with a valid YAML frontmatter block, mdtablefix
preserves the entire block verbatim and formats only the Markdown body that
follows it. A frontmatter block begins with --- as the first line and ends
with --- or ....
This applies to standard CLI formatting as well as options such as --wrap,
--renumber, and --breaks; in particular, --breaks never rewrites the
frontmatter delimiters. The library stream functions provide the same
preservation behaviour. Advanced library callers can use
process_with_frontmatter to run their own body transform without receiving
the frontmatter lines.