Refactor a checkout that does everything.

The checkout in the editor works, and nobody wants to touch it: one class reads two globals, cuts coupon codes out of strings, compares payment names with if, adds up the tax and the fee, and sends the receipt. Refactor it into small parts without changing a single total.

The rules (keep them exactly):

For the order of two notebooks at 120 and four pens at 25, paid by UPI, the total is 397.

Build this API (Line, Settings, Receipt and a FakeNotifier are given):

• NoCoupon(), FlatOff(amount) and PercentOff(percent), each with discount(subtotal), and parse_coupon(code) returning one of them. An unknown code raises ValueError("unknown coupon: GOLD"). This is the only place that reads a code string.
• Card(), Upi() and CashOnDelivery(), each with fee(amount) and a status attribute ("paid" or "pending"); payment_for(name) looks one up and raises ValueError("unknown payment: cheque"); register_payment(name, method) adds one.
• PriceCalculator(settings) with total(lines, coupon, payment): a pure calculation with no mail and no globals.
• CheckoutService(calculator, notifier) with place(lines, coupon_code, payment_name): prices the order, calls notifier.send("receipt: <total>") once, and returns a Receipt. A rejected order sends nothing.

JavaScript and TypeScript use camelCase: parseCoupon, paymentFor, registerPayment, and new Settings({ taxPercent: 10 }). Errors are Errors with the same messages.

The tests check every old total, the calculator with plain numbers and with two different settings side by side, the service with a fake notifier, a new coupon and a new payment method written inside the tests and plugged in, and that no comparison against a payment name is left in your code.