[Invoices]: Implement FacturaeHandler XML generation (Facturae 3.2 — Spain)

[nielsdrost7]

Context

Sub-story of #767. FacturaeHandler (Facturae 3.2 — Spain) currently throws RuntimeException('... not yet implemented ...') instead of generating real XML.

Recommended package

josemmo/facturae-php (MIT, 680k+ installs, 286 stars, 1 open issue, actively maintained — v1.8.5 Aug 2026). Facturae is a genuinely distinct XML schema (not UBL/CII based), so this needs its own dedicated mapping work, unlike the UBL-family formats.

The package also supports XAdES signing and direct submission to Spain's FACe platform — out of scope for "generate the XML" but worth knowing it's there if ever needed.

Scope

Implement FacturaeHandler::generateXml() using josemmo/facturae-php's document builder, mapping our Invoice model fields to the Facturae 3.2 schema.

Acceptance criteria

• FacturaeHandler::generateXml() returns real, well-formed Facturae 3.2 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 Facturae from the list

Sequencing note

Independent of the UBL-family and CII-family sub-stories — clear package, dedicated mapping work. See #767 for full package-research context.