Make the document type configurable, add the preceding invoice reference, allow BT-30 without a scheme

[ahmedm10]

Problem

The document type code (BT-3) is hardcoded to 380 (commercial invoice), marked # TODO: make configurable. Credit notes (381) and corrected invoices (384) can therefore not be written; a cancellation goes out as an invoice with negative amounts, which recipients' software books the wrong way.

A credit note or correction should also refer to the invoice it relates to (BG-3, preceding invoice reference: BT-25 number, BT-26 issue date). There is no way to write that today.

Finally, SpecifiedLegalOrganization/ID (BT-30) always carries a schemeID, defaulting to 0002 (French SIRENE). The scheme is optional in EN 16931 (BT-30-1), and an identifier without a registered scheme — for example a German commercial register entry such as HRB München 123456 — would be mislabelled as a SIRENE number.

Change

• Invoice#type_code (BT-3): written as given, 380 when not set.
• Invoice#preceding_invoice_id (BT-25) and #preceding_invoice_issue_date (BT-26, optional): written as InvoiceReferencedDocument after the monetary summation, where the CII schema places it. Version 2 only, like attachments.
• TradeParty#legal_organization: { id: '...', scheme_id: nil } writes the ID without schemeID. Without the key, 0002 is still the default.

Existing callers get the same XML as before. The one behaviour change: a caller that passes scheme_id: nil explicitly used to get 0002 and now gets no scheme.

Tests

Six new tests in test/invoice_test.rb: the type code defaults to 380; a 381 credit note with the preceding invoice and its date, also validated against the schematron (based on make_eu_invoice, because make_de_invoice does not pass the version 2 schematron on its own); the preceding invoice without a date; no preceding invoice by default; BT-30 without a scheme; BT-30 still defaults to 0002. The full suite passes.