Add innergemeinschaftliche-Lieferung (VAT exemption) support

InvoiceOrder/CorrectionInvoiceOrder gained an optional vatExempt flag
— items already carry 0% rate and de-grossed prices when true (the
caller's responsibility), this only controls display/XML mapping:

- Visual PDF: replaces the "enthält X% MwSt." annotation with
  "Steuerfreie innergemeinschaftliche Lieferung (§4 Nr. 1b UStG)" on
  both the invoice and its Storno/Gutschrift.
- EN16931 XML (buildEInvoiceData.ts): VAT category 'K' instead of 'S'
  on every line/tax-subtotal, plus BT-120/BT-121 exemption reason
  (VATEX-EU-IC) — but only on cac:TaxTotal's own TaxSubtotal.TaxCategory,
  not on InvoiceLine/AllowanceCharge's TaxCategory, which
  @e-invoice-eu/core's schema rejects those same two fields on (caught
  locally before Mustang ever saw it — a fully-rendered PDF/A-3 with a
  hand-inflated CII XML confirmed CategoryCode/ExemptionReasonCode/
  ExemptionReason land correctly).

Also surfaced (not fixed, unrelated to this change): a seller with no
iban configured throws in @e-invoice-eu/core's UBL→CII conversion —
doesn't affect this shop's own production data (iban is always set),
noting for whoever touches this next.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/einvoice/buildEInvoiceData.ts

@@ -23,6 +23,41 @@ type BuyerCountryCode = UblInvoice["cac:AccountingCustomerParty"]["cac:Party"]["
 // summed group uses this same code.
 const VAT_CATEGORY = "S";
 
+// UNTDID 5305 code 'K' — "VAT exempt for EEA intra-community supply of
+// goods and services" — the EN16931 category for an innergemeinschaftliche
+// Lieferung (§4 Nr. 1b UStG / Art. 138 VAT Directive), distinct from 'S'
+// (positive standard rate), 'Z' (zero-rated but still taxable), and 'AE'
+// (domestic reverse charge under §13b UStG — doesn't apply here at all,
+// see the frontend repo's lib/vatExemption.ts for why this shop's checkout
+// only ever produces this 'K' exemption for a cross-border validated B2B
+// sale, never 'AE').
+function vatTaxCategory(rate: number, vatExempt: boolean): Record<string, unknown> {
+  return {
+    "cbc:ID": vatExempt ? "K" : VAT_CATEGORY,
+    "cbc:Percent": String(rate),
+    "cac:TaxScheme": { "cbc:ID": "VAT" },
+  };
+}
+
+// BT-120/BT-121 (VAT exemption reason text/code) are document-level fields
+// that only exist on cac:TaxTotal's own cac:TaxSubtotal.cac:TaxCategory —
+// confirmed empirically: @e-invoice-eu/core's ajv schema rejects
+// TaxExemptionReasonCode/TaxExemptionReason as "additional properties" the
+// moment they're placed on an cac:InvoiceLine's ClassifiedTaxCategory or a
+// cac:AllowanceCharge's TaxCategory (both use their own, more restrictive
+// generated type despite conceptually being "the same" UBL TaxCategory
+// complex type) — caught locally before ever reaching Mustang/CI, exactly
+// the kind of runtime-only-visible constraint this pipeline has hit before
+// (see amtCents()'s own comment). BR-K-10 requires either field whenever
+// category is 'K', satisfied here since this is the one place they're
+// actually allowed to live.
+function vatTaxSubtotalCategory(rate: number, vatExempt: boolean): Record<string, unknown> {
+  return {
+    ...vatTaxCategory(rate, vatExempt),
+    ...(vatExempt ? { "cbc:TaxExemptionReasonCode": "VATEX-EU-IC", "cbc:TaxExemptionReason": "Innergemeinschaftliche Lieferung" } : {}),
+  };
+}
+
 // UN/ECE Recommendation 20 unit-of-measure code for "one" (a countable
 // piece/unit) — this shop sells discrete products (ToDo-Karten sets,
 // notebooks, ...), never anything sold by weight/length/volume, so every
@@ -219,7 +254,7 @@ function computeRateGroups(lines: ComputedLine[], discountAmount: number, shippi
     .sort((a, b) => b.rate - a.rate);
 }
 
-function taxTotal(rateGroups: RateGroup[]): UblInvoice["cac:TaxTotal"] {
+function taxTotal(rateGroups: RateGroup[], vatExempt: boolean): UblInvoice["cac:TaxTotal"] {
   const totalTaxCents = rateGroups.reduce((sum, g) => sum + g.taxCents, 0);
   return [
     {
@@ -227,17 +262,13 @@ function taxTotal(rateGroups: RateGroup[]): UblInvoice["cac:TaxTotal"] {
       "cac:TaxSubtotal": rateGroups.map((g) => ({
         ...amtCents("cbc:TaxableAmount", g.taxableCents),
         ...amtCents("cbc:TaxAmount", g.taxCents),
-        "cac:TaxCategory": {
-          "cbc:ID": VAT_CATEGORY,
-          "cbc:Percent": String(g.rate),
-          "cac:TaxScheme": { "cbc:ID": "VAT" },
-        },
+        "cac:TaxCategory": vatTaxSubtotalCategory(g.rate, vatExempt),
       })),
     },
-  ] as UblInvoice["cac:TaxTotal"];
+  ] as unknown as UblInvoice["cac:TaxTotal"];
 }
 
-function allowanceCharges(rateGroups: RateGroup[]): UblInvoice["cac:AllowanceCharge"] {
+function allowanceCharges(rateGroups: RateGroup[], vatExempt: boolean): UblInvoice["cac:AllowanceCharge"] {
   // Loosely typed here (build as Record<string, unknown>, cast once at the
   // return boundary) — VAT_CATEGORY's literal type widens to plain
   // `string` the moment it's read through an intermediate variable, which
@@ -246,7 +277,7 @@ function allowanceCharges(rateGroups: RateGroup[]): UblInvoice["cac:AllowanceCha
   // exact same widening issue.
   const entries: Record<string, unknown>[] = [];
   for (const g of rateGroups) {
-    const taxCategory = { "cbc:ID": VAT_CATEGORY, "cbc:Percent": String(g.rate), "cac:TaxScheme": { "cbc:ID": "VAT" } };
+    const taxCategory = vatTaxCategory(g.rate, vatExempt);
     if (g.allowanceCents > 0) {
       entries.push({
         "cbc:ChargeIndicator": "false",
@@ -267,7 +298,7 @@ function allowanceCharges(rateGroups: RateGroup[]): UblInvoice["cac:AllowanceCha
   return entries.length ? (entries as unknown as UblInvoice["cac:AllowanceCharge"]) : undefined;
 }
 
-function invoiceLines(lines: ComputedLine[]): UblInvoice["cac:InvoiceLine"] {
+function invoiceLines(lines: ComputedLine[], vatExempt: boolean): UblInvoice["cac:InvoiceLine"] {
   return lines.map(({ item, netCents }, i) => {
     const netUnitPrice = item.unitPrice / (1 + item.taxRatePercent / 100);
     return {
@@ -276,11 +307,7 @@ function invoiceLines(lines: ComputedLine[]): UblInvoice["cac:InvoiceLine"] {
       ...amtCents("cbc:LineExtensionAmount", netCents),
       "cac:Item": {
         "cbc:Name": item.variantName ? `${item.productName} (${item.variantName})` : item.productName,
-        "cac:ClassifiedTaxCategory": {
-          "cbc:ID": VAT_CATEGORY,
-          "cbc:Percent": String(item.taxRatePercent),
-          "cac:TaxScheme": { "cbc:ID": "VAT" },
-        },
+        "cac:ClassifiedTaxCategory": vatTaxCategory(item.taxRatePercent, vatExempt),
       },
       // Unit price, not a summed/reconciled total — kept as the plain
       // (unrounded-to-cent) net unit price for reference; BR-CO-10 only
@@ -345,9 +372,10 @@ function paymentTerms(note: string): UblInvoice["cac:PaymentTerms"] {
 // Full original invoice — one line per order item, positive amounts,
 // InvoiceTypeCode 380 ("Commercial invoice").
 export function buildEInvoiceData(order: InvoiceOrder, seller: InvoiceSeller): Invoice {
+  const vatExempt = Boolean(order.vatExempt);
   const lines = computeLines(order.items);
   const rateGroups = computeRateGroups(lines, order.discountAmount, order.shippingCost);
-  const allowanceChargeEntries = allowanceCharges(rateGroups);
+  const allowanceChargeEntries = allowanceCharges(rateGroups, vatExempt);
   const paid = isPaidImmediately(order.paymentMethodTitle);
 
   return {
@@ -371,13 +399,13 @@ export function buildEInvoiceData(order: InvoiceOrder, seller: InvoiceSeller): I
       // invoice's BT-115 is 0 (see legalMonetaryTotal()'s `prepaid` param).
       ...(paid ? {} : { "cac:PaymentTerms": paymentTerms("Zahlbar sofort ohne Abzug.") }),
       ...(allowanceChargeEntries ? { "cac:AllowanceCharge": allowanceChargeEntries } : {}),
-      "cac:TaxTotal": taxTotal(rateGroups),
+      "cac:TaxTotal": taxTotal(rateGroups, vatExempt),
       // `order.total` is what the customer was actually charged at
       // checkout — any cent of drift against this breakdown's own
       // TaxInclusiveAmount is declared via PayableRoundingAmount inside
       // legalMonetaryTotal(), not silently absorbed into either figure.
       "cac:LegalMonetaryTotal": legalMonetaryTotal(rateGroups, Math.round(order.total * 100), paid),
-      "cac:InvoiceLine": invoiceLines(lines),
+      "cac:InvoiceLine": invoiceLines(lines, vatExempt),
     },
   };
 }
@@ -394,6 +422,7 @@ export function buildEInvoiceData(order: InvoiceOrder, seller: InvoiceSeller): I
 // Stornorechnung-vs-Gutschrift policy (full reversal incl. shipping vs.
 // only returned quantities, no shipping, no discount reproration).
 export function buildCorrectionEInvoiceData(kind: CorrectionInvoiceKind, order: CorrectionInvoiceOrder, seller: InvoiceSeller): Invoice {
+  const vatExempt = Boolean(order.vatExempt);
   const effectiveItems =
     kind === "storno"
       ? order.items
@@ -401,7 +430,7 @@ export function buildCorrectionEInvoiceData(kind: CorrectionInvoiceKind, order:
 
   const lines = computeLines(effectiveItems);
   const rateGroups = computeRateGroups(lines, kind === "storno" ? order.discountAmount : 0, kind === "storno" ? order.shippingCost : 0);
-  const allowanceChargeEntries = allowanceCharges(rateGroups);
+  const allowanceChargeEntries = allowanceCharges(rateGroups, vatExempt);
   // No separately-stored "charged total" exists for a correction event the
   // way `order.total` does for the original invoice — the reversal amount
   // *is* this breakdown's own TaxInclusiveAmount, so PayableAmount is
@@ -440,9 +469,9 @@ export function buildCorrectionEInvoiceData(kind: CorrectionInvoiceKind, order:
       // branch above) — payment terms text satisfies the rule instead.
       "cac:PaymentTerms": paymentTerms("Der Rechnungsbetrag wird erstattet."),
       ...(allowanceChargeEntries ? { "cac:AllowanceCharge": allowanceChargeEntries } : {}),
-      "cac:TaxTotal": taxTotal(rateGroups),
+      "cac:TaxTotal": taxTotal(rateGroups, vatExempt),
       "cac:LegalMonetaryTotal": legalMonetaryTotal(rateGroups, taxInclusiveCents, false),
-      "cac:InvoiceLine": invoiceLines(lines),
+      "cac:InvoiceLine": invoiceLines(lines, vatExempt),
     },
   };
 }