Multi-step completion in a single response

A single response may indicate that more than one step is already completed.

• A step being marked as already completed means that, for the current checkout state and the current basket state, no additional input is required to proceed past that step.

• This evaluation is state-based, not request-based. Client implementations must not assume a strict "one request = one step completion" relationship.

• If the basket content changes (for example: quantity changes, item added/removed), previously completed steps may no longer be considered completed. In that case, the response returns the up-to-date checkout state and indicates the appropriate step to continue.