Uses PyMuPDF with font-size heading detection, running header/footer
removal, table extraction, and optional watermark stripping — the Assembly
PDF→MD pipeline. Returns the Markdown text; when `output_path` is given it
also writes a `.md` file there.

Args:
    pdf_path: Path to the source `.pdf` file.
    output_path: Optional path to also write the Markdown to (`.md`).
    strip_watermarks: Remove Standards-NZ / IHS style watermark lines and
        light-grey overlay text.
    front_matter: Prepend YAML front-matter linking back to the source PDF.

Returns:
    The converted Markdown as a string.

Renders Markdown → HTML → PDF with WeasyPrint (the Assembly MD→PDF stack)
using a clean print stylesheet. Provide EITHER `md_path` OR `markdown_text`.
A PDF is binary, so `output_path` is required.

Args:
    output_path: Path to write the resulting `.pdf` (required).
    md_path: Path to a source `.md` file.
    markdown_text: Raw Markdown string (alternative to `md_path`).
    title: Optional document title (falls back to front-matter `title`).
    css: Optional CSS to replace the built-in print stylesheet.

Returns:
    A confirmation string with the output path, byte size, and page count.