feat: request body digest binding

[brendanjryan]

Summary

Binds the HTTP request body to the payment challenge via SHA-256 digest, preventing replay attacks where a credential issued for one request body is presented with a different body.

Changes

• src/server/Transport.ts: HTTP transport's captureRequest now reads body bytes and computes a bodyDigest (sha-256=...). Added safeBodyBytes helper for graceful body extraction.
• src/server/Mppx.ts: Passes capturedRequest.bodyDigest as the challenge's digest field (HMAC-bound via the 6-slot binding: realm|method|intent|request|expires|digest). After HMAC and scope verification pass, verifies the echoed challenge's digest matches the current request body digest.
• Graceful degradation: GET/bodiless requests omit the digest entirely — no enforcement when both sides have no body.

Verification flow

1. Client sends POST with body → server computes sha-256 digest, includes in challenge HMAC
2. Client echoes challenge back with credential
3. Server recomputes digest from current body, compares to echoed challenge digest
4. Mismatch → 402 with "request body does not match challenge digest"

Tests added

• Challenge includes sha-256=... digest when request has a body
• Challenge omits digest when request has no body
• Rejects credential when body changes between challenge and verification
• Accepts credential when body matches between challenge and verification
• GET request without body still works (no digest)

Stack

PR 3 of 3 in the mppx hardening stack (stacked on PR #299):

1. feat: verified context API #298 Verified context API
2. feat: binding semantics hardening #299 Binding semantics hardening
3. → This PR: Request body digest binding

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/mppx@300

commit: 45a8016

[brendanjryan]

Closing as redundant and not worth the tradeoffs