[Invoices]: Implement ZugferdHandler XML generation (ZUGFeRD)

[nielsdrost7]

Context

Sub-story of #767. ZugferdHandler (ZUGFeRD 1.0/2.0 — 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) — ZUGFeRD and Factur-X are closely aligned CII profiles, same library covers both.

A horstoeko/zugferd-laravel wrapper exists, but its last release (March 2025) is stale versus the base package's August 2026 release — recommend depending on horstoeko/zugferd directly rather than the wrapper.

Scope

Implement ZugferdHandler::generateXml() using horstoeko/zugferd. Reuses the same package as the CII and Factur-X sub-stories — do together or immediately after one another.

Acceptance criteria

• ZugferdHandler::generateXml() returns real, well-formed ZUGFeRD 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 ZUGFeRD from the list

Sequencing note

Shares horstoeko/zugferd with #777 and #778. See #767 for full package-research context.