Model the checkout flow as states and transitions

The Practice Hub checkout is a three-step wizard:

1. Shipping — a shipping-address field, then "Continue to payment"
2. Payment — a payment-reference field, then "Review order"
3. Review — a summary, then "Place order", which calls POST /api/orders

Known server behaviour: POST /api/orders returns 409 EMPTY_CART if the cart has no lines, and requires an Idempotency-Key header.

Your task: draw the state model, then derive the test cases — including the ones that matter most, which are the transitions the UI does not offer. Then answer: "Which single test would you keep if you had time for one?"