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/codebase/architecture.md

@@ -288,6 +288,20 @@ tests/
 
 ---
 
+## Phase 126 — Invoice GUS Field Mapping (KRS heuristic)
+
+### MfWhitelistApiClient (`src/Core/Http/MfWhitelistApiClient.php`)
+- `lookupByNip()` zwraca dodatkowo `krs: string` i `is_jdg: bool` (true gdy `subject.krs === ''`). Pozostaly kontrakt bez zmian.
+- Heurystyka: JDG = brak KRS w MF. Spolka = `krs` niepuste. Pattern do reuse w przyszlych formularzach opartych o NIP lookup.
+
+### InvoiceController::nipLookup (`/api/nip/lookup`)
+- JSON `data` rozszerzony o `is_jdg: bool`. Konsumowane przez JS w `accounting/invoice_form.php`.
+
+### invoice_form.php JS — warunkowe mapowanie pola docelowego
+- `d.is_jdg=true` (JDG): MF `name` (osoba fizyczna) -> `#buyer_name` (Imie i nazwisko). `#buyer_company_name` nie ruszane (pre-fill z `order_addresses.name` zachowany — czesto trzyma pelna nazwe firmy JDG).
+- `d.is_jdg=false` (spolka): MF `name` (legal name) -> `#buyer_company_name`. `#buyer_name` nie ruszane (pre-fill z zamowienia — np. osoba kontaktowa).
+- Pola adresowe (street/postal_code/city) zawsze nadpisywane.
+
 ## Phase 116 - HostedSMS Integration Settings
 
 ### HostedSmsIntegrationRepository (`src/Modules/Settings/HostedSmsIntegrationRepository.php`)