feat(stage-10c): Square Mobile POS / Checkout
- Add PaymentMethod enum (cash, card) with label/iconHint extensions
- Add PaymentResult value object (transactionId, amountCents, method, completedAt, receiptUrl, amountFormatted)
- Add PaymentService abstract contract + PaymentException
- Add CheckoutController (ChangeNotifier) with CartEntry and sealed CheckoutState hierarchy
- Add FakePaymentService (always succeeds, counter-based IDs)
- Update feature_orders barrel exports
- Add MobileCheckoutPage (product selection -> cart -> payment -> receipt flow)
- Add Sales tab (index 4) to MobileShell; 6 tabs total
- Wire paymentService + checkoutCatalogue into all MobileAppServices factories
- Add 36 new feature_orders tests (234 total)
- Add 18 new kell_mobile tests (72 total); 1373 total across all packages
- Update widget_test.dart: destination count 5 -> 6
- Update build_execution_tracker.md and master_development_brief.md
All tests passing. dart analyze clean.
Mike Kell
47b13b1ed1