Phase 4: Mustang EN16931/PDF-A-3 validation in CI

Adds a Gitea Actions workflow that generates realistic e-invoice fixtures (multi-VAT-rate original invoice, Storno, Gutschrift) and validates them against the reference Mustang validator on every push, catching a broken EN16931 mapping before it reaches production instead of relying on manual spot-checks.

README.md

@@ -19,6 +19,8 @@ One canonical implementation, consumed by both repos, makes this class of drift
 
 **2026-07-23, Phase 3: actual e-invoicing.** `renderInvoiceEInvoice()`/`renderCorrectionInvoiceEInvoice()` (in `einvoice/`) produce a ZUGFeRD/Factur-X hybrid PDF/A-3 with an embedded EN16931 XML instead of a plain PDF — see "E-invoicing" below.
 
+**2026-07-23, Phase 4: CI validation.** `.gitea/workflows/validate-einvoice.yml` runs on every push/PR (via git.mk360.de's own self-hosted Gitea Actions runner) — see "CI validation (Mustang)" below.
+
 ## How this is consumed
 
 Not published to npm — installed as a git dependency:
@@ -51,3 +53,12 @@ Ships raw TypeScript/TSX source (no build step) via `main`/`types` pointing stra
   - Payment means: included whenever `seller.iban` is set (matching the visual PDF footer's own "always show it" behavior since Phase 2), with a `PaymentMeansCode` mapped from the order's actual `paymentMethodTitle` (`Überweisung` → `30` credit transfer, `Kreditkarte` → `48`, `PayPal` → `68`, anything unrecognized → `1` "Instrument not defined" — a payment method added in Payload doesn't need a matching code deploy here to keep e-invoice generation working).
 - **`einvoice/countryCode.ts`** — `sellerCountry`/`order.country` are free text ("Deutschland"), not an ISO-3166 select field, but EN16931 wants a fixed two-letter code. Small closed mapping (DACH region only, this shop's actual shipping footprint), falling back to `DE`.
 - **Library: `@e-invoice-eu/core`**, format `'Factur-X-EN16931'` (the ZUGFeRD "Comfort" profile, the minimum EN16931-compliant level). Verified by actually generating a sample invoice from `SAMPLE_INVOICE_ORDER` and inflating the embedded XML stream out of the resulting PDF/A-3 by hand (the library ships no attachment-reading API of its own to check this against) — confirmed correct `CrossIndustryInvoice` XML, EN16931 guideline reference, per-rate tax breakdown, and payment means, not just "it didn't throw."
+
+## CI validation (Mustang)
+
+Every push/PR runs `.gitea/workflows/validate-einvoice.yml` against git.mk360.de's own self-hosted Gitea Actions runner (`vps-runner`, registered on the same VPS as Gitea/Payload — see `/home/marco/dev/docker/docker-compose.yml`'s `act_runner` service):
+
+1. `npm run fixtures:einvoice` (`scripts/generate-einvoice-fixtures.mts`) renders 3 PDFs into `.mustang-fixtures/` (gitignored, regenerated every run) — an original invoice with **two simultaneous VAT rates (19%+7%) plus a discount and shipping cost together** (the combination `SAMPLE_INVOICE_ORDER` doesn't cover), plus a Storno and a Gutschrift against the same order.
+2. [Mustang-CLI](https://www.mustangproject.org/commandline/) (the reference ZUGFeRD/Factur-X validator, `--action validateExpectValid -d .mustang-fixtures`) checks all 3 for EN16931 + PDF/A-3 conformance in one call. Non-zero exit fails the job. The jar is downloaded pinned to a specific release + sha256 (`core-2.24.0`) rather than a floating `latest` tag, right in the workflow — no Docker image for Mustang is actively maintained by the upstream project itself, so downloading the jar directly into a `setup-java` step was simpler and more trustworthy than depending on a third-party wrapper image.
+
+Run the same check locally with `npm run fixtures:einvoice`, then point a locally-downloaded `Mustang-CLI-*.jar` at `.mustang-fixtures/` yourself — useful for iterating on `buildEInvoiceData.ts` without waiting on a CI round-trip.