Design a supermarket checkout system

Design the software for a supermarket checkout system. A grocery store has multiple checkout lanes operated by cashiers, and the design should also be extensible to self-checkout. Customers bring items to a register, where the cashier scans products and the system computes the final total. The system should support: - product lookup by barcode or PLU code - unit-based and weight-based items - quantities for the same item - taxes, discounts, promotions, and coupons - a running subtotal while items are scanned - voiding an item before payment - multiple payment methods such as cash and card - receipt generation - opening and closing a register session for a cashier Discuss: - the core objects/classes and their responsibilities - the main APIs or service boundaries - the checkout session lifecycle and state transitions - how pricing and promotions should be modeled cleanly - error handling and data consistency concerns - how to extend the design later for loyalty programs, returns, and split payments