docs: add MPP and x402 interop guide

[EfeDurmaz16]

Summary

• add an ACK-Pay interoperability guide for MPP and x402
• map ACK-Pay Payment Requests, Payment Options, Payment Services, execution proofs, and ACK Receipts to MPP/x402 concepts
• document HTTP 402, MCP paid-tool, policy-before-signing, and receipt evidence considerations

Closes #90.

Verification

• pnpm exec oxfmt --check docs/ack-pay/mpp-x402-interop.mdx docs/docs.json
• pnpm exec mintlify broken-links

Summary by CodeRabbit

Documentation

• Added new ACK-Pay interoperability documentation detailing how ACK-Pay concepts map to MPP and x402 payment flows. Content includes HTTP 402 request-response flow patterns, payment requirement handling, policy verification checklists, receipt evidence format specifications, MCP tool payment handling, guidance for implementing transport integrations, and planned future improvements.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 00d13103-a5cf-403c-9d86-5f9d80b79ea2

📥 Commits

Reviewing files that changed from the base of the PR and between c83663f and 4bfdef2.

📒 Files selected for processing (1)

• docs/ack-pay/mpp-x402-interop.mdx

Walkthrough

This PR adds comprehensive documentation mapping ACK-Pay concepts to MPP and x402 payment protocols. A concept-mapping table correlates ACK primitives with their MPP and x402 equivalents. The document describes HTTP 402 flows, MCP tool payment patterns, integration policy requirements, receipt evidence structure, and implementation guidance. The docs navigation is updated to surface the new page.

Changes

ACK-Pay / MPP / x402 Interoperability Documentation

Layer / File(s)	Summary
Concept foundations and mapping table docs/ack-pay/mpp-x402-interop.mdx	Document metadata, scope framing, and concept-mapping table correlating ACK-Pay Payment Request/Option/Service/Execution proof/Receipt with MPP and x402 equivalents, establishing rail-neutral composition.
HTTP 402 and MCP payment flows docs/ack-pay/mpp-x402-interop.mdx	HTTP 402 flow sequence from server payment advertisement through client selection, Payment Service execution with evidence recording, Receipt issuance, and verification. MCP Tool Payments section preserves ACK identity and Receipt semantics with MPP-like transport.
Integration policy, receipt evidence, and implementation guidance docs/ack-pay/mpp-x402-interop.mdx	Policy Before Signing checklist for payment execution constraints. Receipt Evidence guidance on stable field references (protocol, network, reference, service, facilitator, policy hash). Implementation points on ACK field stability, replay resistance, and credential exposure prevention. Future Work enumerates follow-up documentation, profiles, demos, and conformance tests.
Documentation navigation configuration docs/docs.json	Registers new ack-pay/mpp-x402-interop page under ACK-Pay documentation tab.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'docs: add MPP and x402 interop guide' clearly and concisely summarizes the main change: adding documentation about ACK-Pay interoperability with MPP and x402 protocols.
Linked Issues check	✅ Passed	The PR fully implements the core objectives from issue #90: includes a mapping table between ACK-Pay/MPP/x402 concepts, documents composition points, provides guidance on opaque extension metadata, and delivers a non-normative documentation page as requested.
Out of Scope Changes check	✅ Passed	All changes are within scope: the new mpp-x402-interop.mdx documentation file and docs.json navigation update directly address the issue #90 requirements with no unrelated modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/ack-pay/mpp-x402-interop.mdx`:
- Line 23: Update the "Payment execution proof" row in the table that currently
maps to "Credential" and "Payment payload" to also include "Payment-Signature"
as part of the x402 execution-proof mapping; locate the table row containing the
exact cell text "Payment execution proof" and add "Payment-Signature" alongside
"Payment payload" in the corresponding mapping cell so the table reflects
"PaymentPayload / Payment-Signature" as required by Issue `#90`.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 1e2cf717-690a-400c-abad-ad25c29cd82a

📥 Commits

Reviewing files that changed from the base of the PR and between 6f65936 and c83663f.

📒 Files selected for processing (2)

• docs/ack-pay/mpp-x402-interop.mdx
• docs/docs.json