One-Call Checkout (Facade Pattern)

Checking out an order really means three separate jobs — reserving stock, charging a card, and booking a shipment — each with its own failure mode. The Facade pattern hides that messy orchestration behind one friendly call. Build a CheckoutFacade over in-memory Inventory, Payment, and Shipping subsystems, add rollback when a step fails, return a tracking id on success, keep an order log, and finish with a one-argument quick_buy convenience method.