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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.1.0",
+  "version": "0.2.0",
   "private": true,
   "description": "Shared invoice / correction-invoice (Stornorechnung, Gutschrift) PDF generation and VAT-breakdown math, consumed as a git dependency by both the einfach-produktiv frontend and the payload backend — not published to npm.",
   "type": "module",