feat(126): invoice GUS field mapping fix (JDG/KRS heuristic)

MfWhitelistApiClient.lookupByNip() exposes is_jdg/krs from MF Biala Lista.
InvoiceController.nipLookup propagates is_jdg in JSON response.
invoice_form.php JS conditionally targets buyer_name (JDG) or
buyer_company_name (spolka z KRS); second field keeps zamowienie pre-fill.

Fixes apparent field swap on /orders/{id}/invoice/create after GUS lookup
for JDG (sole trader) where MF returns natural person in subject.name.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.paul/ROADMAP.md

@@ -25,6 +25,7 @@ Wystawianie faktur dla klientow z NIP poprzez integracje z Fakturownia (app.fakt
 | 123 | Receipts Export VAT Breakdown | 1/1 | Complete (2026-05-12; manual XLSX smoke pending operator) |
 | 124 | SMS Templates | 1/1 | Complete (2026-05-13; migration + manual SMS smoke pending operator) |
 | 125 | invoice_requested Import Fix (shopPRO+Allegro NIP detection, drop legacy is_invoice column) | 1/1 | Complete (2026-05-13; migration + manual smoke pending operator) |
+| 126 | Invoice GUS Field Mapping Fix (KRS-based heuristic: JDG → name do "Imię i nazwisko", spółka → "Nazwa firmy") | 1/1 | Complete (2026-05-13; manual smoke pending operator) |
 
 Planowane kolejne fazy v3.7 (kandydaci, do rozplanowania):
 - Eksport XLSX listy wystawionych faktur (analogicznie do paragonow)
@@ -507,4 +508,4 @@ Archive: `.paul/milestones/v0.1-ROADMAP.md`
 
 ---
 *Roadmap created: 2026-03-12*
-*Last updated: 2026-05-13 - Phase 125 UNIFY closed*
+*Last updated: 2026-05-13 - Phase 126 UNIFY closed*