List landing page checkout shipping options

Page-scoped public endpoint used by Scalev.checkout.shippingOptions(payload). It accepts items, destination, and payment_method, then resolves the store from the authenticated page. For an authorized published builder page, postcode collection follows that page's store configuration. A postcode remains required for authored HTML pages. For the current published builder form only, allow_incomplete: true also permits quoting shipping before a required postcode is selected. This preview does not authorize order creation or bypass the form's postcode validation. Other browser parameters cannot override the postcode policy. The destination location, current items, shipping availability and quoted costs are still validated by the backend. Published builder forms follow the existing landing-page draft order policy: insufficient inventory does not block shipping previews. With both allow_incomplete: true and include_provisional: true, the current published builder can quote shipping while a customer-entered price is unfinished. The result is non-payable and never authorizes an order. Storefront and authored HTML validation remains unchanged; order creation stays strict.