Discriminated Unions

type Circle = { kind: 'circle'; radius: number }
type Rectangle = { kind: 'rectangle'; width: number; height: number }
type Shape = Circle | Rectangle

1. Replace the placeholder ApiState with a discriminated union on status. Valid variants (each exclusive — a loading state must not require success or error fields, and so on):
   • loading — only the loading discriminant
   • success — string-array data
   • error — string error
2. Replace the placeholder PaymentMethod with a discriminated union on type. Valid variants (each exclusive):
   • credit card — last4 and expiry strings
   • PayPal — email string
   • bank — accountNumber string
3. Remove the @ts-expect-error comments once the unions are correct so the never exhaustiveness checks type-check
4. Export renderState and describePayment by name (their bodies are already written)

Completion criteria

• Named exports: renderState, describePayment
• renderState / describePayment keep working for each variant using the return strings already in the starter switch cases
• Invalid mixed shapes (for example success fields on a loading state) are not representable