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

Problem/Motivation

The payment interaction landing page (orchestra_payment PaymentInteraction) shows only the amount due. A consumer often wants to show what is being paid for there: an itemized summary (for a booking, the order grouped by event with each ticket and price). Today the Payable value object carries no way to provide that, and PaymentInteraction is deliberately domain-neutral, so there is no seam for it.

Proposed resolution

Add 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; when unset, the page is unchanged. The resolver (the existing domain seam) owns what the summary is, so orchestra_payment stays payment-agnostic and learns nothing about the consumer domain. The change is additive and backward compatible (the field defaults to NULL).

Remaining tasks

• Add the optional summary render array to the Payable value object.
• Render it in PaymentInteraction landing page when present.
• Add a kernel test.

Consumer

A consumer (a booking order resolver) populates the summary in its own issue.