[Invoices]: Implement FacturXHandler XML generation (Factur-X)

[nielsdrost7]

Context

Sub-story of #767. FacturXHandler (Factur-X 1.0 — France/Germany) currently throws RuntimeException('... not yet implemented ...') instead of generating real XML.

Recommended package

horstoeko/zugferd (MIT, actively maintained — v1.0.132 as of Aug 2026). Factur-X (CII XML + PDF/A-3 embedding) is this library's primary use case.

Scope

Implement FacturXHandler::generateXml() using horstoeko/zugferd. Reuses the same package as the CII sub-story — do together or immediately after one another to amortize integration work.

Acceptance criteria

• FacturXHandler::generateXml() returns real, well-formed Factur-X CII XML
• Output passes PeppolXmlValidator
• Unit test asserts well-formedness and key fields, following the PeppolBisHandlerTest pattern
• FormatHandlersTest's unimplemented-formats test is updated to drop Factur-X from the list

Sequencing note

Shares horstoeko/zugferd with #777 and #779. See #767 for full package-research context.