drafthorse and factur-x check the XSD only: 12 of 13 common invoice mistakes pass, EN 16931 rejects them (measured)

serialize(schema=...) and xml_check_xsd() validate the shape of a Factur-X/ZUGFeRD file, not the EN 16931 business rules. Measured with KoSIT, plus a 15-line saxonche helper for pytest.