Let a PayableResolver attach a rendered summary to the payment landing page

Adds an optional summary render array to the Payable value object. When a PayableResolver sets it, PaymentInteraction renders it on the landing page above the amount due; unset leaves the page unchanged. The resolver owns the summary content, so orchestra_payment stays payment-agnostic. Additive and backward compatible. The example resolver supplies a demo summary and PaymentBridgeTest covers the rendering.