Compute a page runtime checkout summary

Page-scoped public endpoint used by Scalev.checkout.estimateSummary(payload). It accepts items, destination, payment_method, shipping_option, and an optional discount_code_code. It recomputes shipping and applies backend-owned Other Income and service fees for display. The current Store settings are the authoritative customer-fee policy; clients do not submit that policy or a fee confirmation ID. Authorized Builder pages use this same runtime API. With allow_incomplete: true, missing delivery selections return an explicitly incomplete product subtotal, never a payable total. For the current published builder form, an unselected postcode alone does not prevent a shipping quote; the form and order endpoint still enforce postcode collection. Authored HTML and storefront rules stay strict. With include_breakdown: true, the response also includes backend-owned receipt lines and discount amounts. Equal-price occurrences of the same variant across selected items and bundles share one receipt line with their combined quantity, retaining first occurrence order. Distinct catalog identities or unit prices remain separate. These flags default to false; omitting them preserves the existing complete-summary response. Published builder forms follow the landing-page draft order policy: insufficient stock alone does not make a quote incomplete. A quote is not inventory authorization; order creation and status transitions enforce the existing inventory policy. Storefront and authored HTML stock checks remain unchanged.