docs(ack-pay): document optional receipt evidence metadata

[EfeDurmaz16]

Context

ACK receipts already bind a payment receipt credential to a paymentRequestToken and paymentOptionId. For more advanced agent-commerce flows, it would be useful to document a narrow, optional metadata shape for evidence and policy references without changing the core receipt VC format.

This is especially relevant for:

• MPP receipts and payment sessions
• x402 signed receipts / payment response references
• AP2-style mandate or authorization chains
• policy-before-signing demos
• audit trails for agent actions

Proposal

Define a non-normative receipt metadata example/profile with fields such as:

type PaymentEvidenceMetadata = {
  policyRef?: string
  policySnapshotHash?: string
  mandateRef?: string
  executionRef?: string
  executionReceiptHash?: string
  settlementNetwork?: string
  settlementReference?: string
}

The field names are placeholders; the useful part is a documented pattern for linking an ACK receipt to the policy/mandate/execution evidence that caused the payment.

Why this helps

• Keeps the core PaymentReceiptCredential small.
• Gives implementers a safe extension point for auditability.
• Helps ACK compose with MPP/x402/AP2 without forcing one protocol's fields into the ACK core schema.
• Makes receipt verification examples more useful for real agent systems.

Small first slice

Add a docs example plus one test that creates a receipt with metadata and verifies that metadata survives signing/parsing/verification round-trip.

This should avoid changing required schema fields or verifier semantics.