Function build_pdf 

pub(crate) fn build_pdf(
    graph: &PageGraph,
    metadata: &PdfMetadata,
) -> Result<(Vec<u8>, Vec<Diagnostic>)>

Build the PDF bytes from graph. Pulled out of emit so tests can round-trip without touching the filesystem. Returns the bytes plus any encoding diagnostics (currently MOS0032 for Base14 /Differences overflow). Kept pub(crate); the public surface is emit.

Errors

Returns an error if font subsetting fails for any embedded face (only with corrupted font data; the bundled cuts have been verified).