Accepts a PDF and a CII XML, returns a Factur-X compliant PDF/A-3 with the XML embedded as `factur-x.xml`. The XML is validated with the full rule set of `/v1/validate/facturx` (schema + business rules) before embedding; pass `skipValidation=true` for packaging-only mode, which checks structure (profile XSD) but not business rules. For invoices bound for the French exchange network, `rules=br-fr` additionally validates the XML against the BR-FR Flux 2 mandate rules before embedding, exactly as `/v1/validate/facturx` does.