Add Kleinunternehmerregelung (§19 UStG) support

InvoiceOrder/CorrectionInvoiceOrder gain an optional order-level
kleinunternehmer flag, snapshotted per order (same pattern as vatExempt)
so a later toggle of the tenant's setting never rewrites an
already-issued invoice. Takes precedence over vatExempt.

- Visual PDF: "enthält X% MwSt." becomes the §19 UStG notice.
- E-invoice XML: new UNTDID 5305 category "E" with a free-text exemption
  reason (no VATEX code — §19 UStG has none, BR-E-10 allows text alone).
- New Mustang CI fixture + buildEInvoiceData unit test for the "E" path.

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

README.md

@@ -27,6 +27,8 @@ One canonical implementation, consumed by both repos, makes this class of drift
 
 **2026-07-23, Netto row.** A dedicated "Netto" row (Gesamt minus every rate's own tax, summed) now sits between Gesamt and the "enthält X% MwSt." annotation, on every invoice type this package renders — original, Storno, and Gutschrift alike. First version skipped it on `vatExempt` orders (net and Gesamt are the same figure there, so it looked redundant) — corrected same day: it's shown unconditionally, for layout consistency across every invoice, not gated on the tax rate.
 
+**2026-07-24, Kleinunternehmerregelung (§19 UStG).** `InvoiceOrder`/`CorrectionInvoiceOrder` gained optional `kleinunternehmer`, a third order-level VAT treatment alongside `vatExempt` — see "Kleinunternehmerregelung" below. Takes precedence over `vatExempt` wherever both would otherwise apply (a Kleinunternehmer never charges VAT to begin with, so there's nothing left to "exempt" via the separate intra-community rule).
+
 ## How this is consumed
 
 Not published to npm — installed as a git dependency:
@@ -55,10 +57,10 @@ Ships raw TypeScript/TSX source (no build step) via `main`/`types` pointing stra
 - **`einvoice/buildEInvoiceData.ts`** — maps `InvoiceOrder`/`CorrectionInvoiceOrder` + `InvoiceSeller` into the raw UBL-shaped `Invoice` object `@e-invoice-eu/core` expects (the library converts UBL → CII internally for Factur-X output — this package only ever builds the UBL shape, regardless of target format). Reuses `computeTaxBreakdown()` for the per-rate VAT grouping, same as the visual PDFs — one tax-math implementation feeding both the human-readable and machine-readable side of the same document.
   - Every EN16931 amount field turned out, at runtime (via the library's own ajv JSON-schema validation — **not visible in its TypeScript types at all**), to require a sibling `*@currencyID` key the moment the amount itself is present, and every quantity a `*@unitCode`. `amt()`/`qty()` return both keys at once via object spread so a call site can't add one without the other — found by actually running a sample invoice through `generate()` and reading the ajv errors, not from the library's own docs.
   - Original invoice: `InvoiceTypeCode` `380` ("Commercial invoice"). Correction invoice: `381` ("Credit note") — this library has no separate credit-note type, same `Invoice` shape either way, just the type code — with a `cac:BillingReference` pointing back at the original invoice number. Amounts stay positive either way (the credited amount, not a negative number) — EN16931/UBL convention puts the polarity in the type code, not the sign; the PDF's own visual "-{amount}" is a *display* convention layered on top (`correctionInvoicePdf.tsx`'s own `groupByTaxRate()`), not something this XML mapper re-derives independently.
-  - VAT category is `S` ("Standard rated") for any positive-rate order — 19% and 7% both use `S`, with the actual percentage in `cbc:Percent`. When `order.vatExempt` is set (innergemeinschaftliche Lieferung, §4 Nr. 1b UStG), category `K` ("VAT exempt for EEA intra-community supply of goods and services") is used instead, with a `VATEX-EU-IC` exemption reason (BT-120/BT-121) — but **only** on `cac:TaxTotal`'s own `TaxSubtotal.TaxCategory`, never on an `InvoiceLine`'s `ClassifiedTaxCategory` or an `AllowanceCharge`'s own `TaxCategory` — `@e-invoice-eu/core`'s generated ajv schema rejects `TaxExemptionReasonCode`/`TaxExemptionReason` as "additional properties" on those two despite them being conceptually the same UBL `TaxCategory` complex type. Caught locally (a full PDF/A-3 render + hand-inflated CII XML inspection) before ever reaching Mustang. This shop has no domestic reverse-charge (`AE`) sales — only the one cross-border-EU exemption case exists.
+  - VAT category is `S` ("Standard rated") for any positive-rate order — 19% and 7% both use `S`, with the actual percentage in `cbc:Percent`. When `order.vatExempt` is set (innergemeinschaftliche Lieferung, §4 Nr. 1b UStG), category `K` ("VAT exempt for EEA intra-community supply of goods and services") is used instead, with a `VATEX-EU-IC` exemption reason (BT-120/BT-121). When `order.kleinunternehmer` is set (§19 UStG, takes precedence over `vatExempt`), category `E` ("Exempt from tax") is used with a free-text exemption reason only — §19 UStG is a national provision with no EU-wide VATEX code, and BR-E-10 accepts the reason text alone without a code. Both `K`/`E` fields live — but **only** on `cac:TaxTotal`'s own `TaxSubtotal.TaxCategory`, never on an `InvoiceLine`'s `ClassifiedTaxCategory` or an `AllowanceCharge`'s own `TaxCategory` — `@e-invoice-eu/core`'s generated ajv schema rejects `TaxExemptionReasonCode`/`TaxExemptionReason` as "additional properties" on those two despite them being conceptually the same UBL `TaxCategory` complex type. Caught locally (a full PDF/A-3 render + hand-inflated CII XML inspection) before ever reaching Mustang. This shop has no domestic reverse-charge (`AE`) sales — only these two exemption cases exist, and they're mutually exclusive (see `VatMode` in `buildEInvoiceData.ts`).
   - 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). **Known gotcha, not yet fixed**: when `seller.iban` is unset, `cac:PaymentMeans` is simply omitted (`undefined`) — but `@e-invoice-eu/core`'s UBL→CII conversion then throws (`Cannot read properties of undefined (reading 'length')` inside its own `format-cii.service.ts`), unrelated to anything in this package's own code. Doesn't affect this shop's production data (this tenant's `company-settings.iban` is always set), but would break e-invoice generation entirely for any future tenant/order whose seller has no IBAN configured — worth fixing (always pass an empty array rather than `undefined`?) before that ever happens.
 - **`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." Same manual-inflation verification repeated for the `vatExempt` path (2026-07-23) — confirmed `CategoryCode>K`, `ExemptionReasonCode>VATEX-EU-IC`, `ExemptionReason>Innergemeinschaftliche Lieferung` all present in the actual embedded XML, not just the pre-conversion UBL JSON.
+- **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." Same manual-inflation verification repeated for the `vatExempt` path (2026-07-23) — confirmed `CategoryCode>K`, `ExemptionReasonCode>VATEX-EU-IC`, `ExemptionReason>Innergemeinschaftliche Lieferung` all present in the actual embedded XML, not just the pre-conversion UBL JSON. The `kleinunternehmer` path (2026-07-24) is covered by `__tests__/buildEInvoiceData.test.ts` (asserts the pre-conversion UBL `TaxCategory` shape: `E` + the §19 reason text, no code) and by its own Mustang CI fixture (`kleinunternehmer-invoice.pdf`, see "CI validation" below) — not yet manually re-verified against the hand-inflated CII XML the way `K` was; worth doing once this ships to a real Kleinunternehmer tenant.
 
 ## VAT exemption
 
@@ -69,11 +71,20 @@ Ships raw TypeScript/TSX source (no build step) via `main`/`types` pointing stra
 
 See the frontend repo's own README ("VAT exemption" section) for the actual VIES lookup, de-grossing math, and checkout UI this feeds from, and the Payload backend's README ("B2B checkout & VAT exemption") for the persisted `Orders.vatExempt`/`vatIdValidatedAt` fields and audit-trail reasoning.
 
+## Kleinunternehmerregelung (§19 UStG)
+
+`InvoiceOrder.kleinunternehmer`/`CorrectionInvoiceOrder.kleinunternehmer` (optional, default falsy) — a **seller-level**, not order-level, business fact (whether this tenant is a small business under §19 UStG), but still snapshotted onto each order at checkout time (mirroring how `vatExempt` is already frozen there) rather than read live off the seller when an invoice renders. This matters for a reason `vatExempt` doesn't have to worry about: `vatExempt` is inherently decided per order (a live VIES lookup against that specific sale), so there's no "past" value to protect. `kleinunternehmer` is a standing tenant setting that can be toggled on/off at any time — without the snapshot, a tenant switching it later would rewrite the tax treatment of every already-issued invoice the next time it's re-rendered (e.g. a customer's "Rechnung erneut herunterladen"), which is both legally wrong (the treatment at the moment of sale is what counts) and silent. The consuming frontend's checkout is the only place this ever gets read live, from its own `company-settings.kleinunternehmer` — see that repo's README.
+
+Unlike `vatExempt`, this package does **not** expect `unitPrice` to be de-grossed for a `kleinunternehmer` order — a Kleinunternehmer never charged VAT in the first place, so the catalog gross price and the actual net charge are the same figure; only `taxRatePercent` becomes `0` on every item. Display:
+
+- Visual PDF (`invoicePdf.tsx`/`correctionInvoicePdf.tsx`): the "enthält X% MwSt." annotation under Gesamt becomes "Gemäß § 19 UStG wird keine Umsatzsteuer berechnet." instead — takes precedence over the `vatExempt` note if (implausibly) both were ever set.
+- E-invoice XML (`buildEInvoiceData.ts`): VAT category `E` + a free-text exemption reason, no VATEX code — see "E-invoicing" above.
+
 ## 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.
+1. `npm run fixtures:einvoice` (`scripts/generate-einvoice-fixtures.mts`) renders 4 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), a Storno and a Gutschrift against that same order, and a separate single-rate **Kleinunternehmer** original invoice (`kleinunternehmer-invoice.pdf`, every item at 0%, category `E`) — its own fixture rather than a variant of the first, since a real Kleinunternehmer order never has a positive catalog rate to begin with.
+2. [Mustang-CLI](https://www.mustangproject.org/commandline/) (the reference ZUGFeRD/Factur-X validator, `--action validateExpectValid -d .mustang-fixtures`) checks all 4 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.