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.

package.json

@@ -19,7 +19,8 @@
   "scripts": {
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
-    "lint": "eslint ."
+    "lint": "eslint .",
+    "fixtures:einvoice": "tsx scripts/generate-einvoice-fixtures.mts"
   },
   "peerDependencies": {
     "@react-pdf/renderer": "^4.0.0",
@@ -31,6 +32,7 @@
     "@types/react": "^19",
     "eslint": "^9",
     "react": "^19.2.4",
+    "tsx": "^4.19.2",
     "typescript": "^5",
     "vitest": "^4.1.10"
   },