docs(ack-pay): map ACK-Pay concepts to MPP and x402 #90

Context

ACK-Pay already has the right high-level primitives for agent commerce: a server-issued payment request, client/payment-service execution, and a verifiable receipt. Since MPP and x402 are both evolving quickly around HTTP 402 flows, it would be useful to document how ACK-Pay maps to those protocols without making ACK depend on either one.

Current protocol anchors:

• MPP models HTTP payment flows as Challenge -> Credential -> Receipt, with HTTP, MCP/JSON-RPC, and WebSocket transports.
• x402 models PaymentRequired -> PaymentPayload/Payment-Signature -> Payment Response, with schemes such as exact, upto, and batch settlement.
• ACK-Pay models PaymentRequest -> payment execution -> PaymentReceiptCredential.

Proposal

Add a small docs/spec page or appendix that maps ACK-Pay concepts to MPP and x402 concepts:

The page should be explicitly non-normative at first: it would explain where ACK can compose with MPP/x402, what ACK adds beyond rail-specific payment execution, and what fields should remain opaque extension metadata.

Why this helps

• ACK can stay rail-neutral while giving builders a clear migration/interoperability path.
• MPP is adding session, subscription, card, stablecoin, MCP and WebSocket transport surfaces.
• x402 is adding multiple schemes, signed offers/receipts, idempotency, batch settlement and MCP examples.
• Catena's own roadmap already calls out x402 interop and broader payment protocol interoperability.

Small first slice

I would start with docs only:

• docs/ack-pay/operational-considerations.mdx or a new docs/ack-pay/interoperability.mdx
• one diagram or table mapping ACK-Pay <-> MPP <-> x402
• one short example showing an ACK receipt carrying an MPP/x402 execution reference in metadata without changing the core VC shape

No SDK dependency is needed for the first slice.