FH-1.2 (partial): reject non-https bank redirect URLs before trust-bypass
rendering them into the payment popup iframe; show a visible payment
error instead. Popup-vs-navigate-current-tab redesign still open, gated
on a product decision about the return-URL contract.
FH-1.4 (frontend half): one crypto.randomUUID() idempotency key per
checkout attempt, generated when the payment popup opens and reused
across retryPayment(), sent as metadata.idempotencyKey - the only
extensibility point @marketplaces/payment's PaymentRequest exposes.
FH-0.2: confirmed done - createPayment(payload, headers) has zero
callers, cart.component.ts already runs entirely through the
server-priced checkout path.
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