[Invoices]: Implement CiiHandler XML generation (UN/CEFACT CII)

[nielsdrost7]

Context

Sub-story of #767. CiiHandler (UN/CEFACT Cross Industry Invoice) 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). Handles native CII syntax directly, which is the base format ZUGFeRD/Factur-X profile rules layer on top of.

Note: the package's own maintainers point new projects at horstoeko/invoicesuite as an eventual successor, but it's not yet mature enough (v0.0.32) to depend on here.

Scope

Implement CiiHandler::generateXml() using horstoeko/zugferd's CII document builder.

Acceptance criteria

• CiiHandler::generateXml() returns real, well-formed 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 CII from the list

Sequencing note

Shares horstoeko/zugferd with #778 and #779 (Factur-X and ZUGFeRD sub-stories) — recommend doing these three together or back-to-back to amortize the integration work. See #767 for full package-research context.