Add deferred payment scheme and Cloudflare network specification

[CameronWhiteside]

Description

Introduces the deferred payment scheme for x402, enabling instant resource access through cryptographically signed payment commitments with deferred settlement through a trusted network serving as Merchant of Record.

What's Included

File	Description
specs/schemes/deferred/scheme_deferred.md	Core deferred scheme definition
specs/schemes/deferred/scheme_deferred_cloudflare.md	Cloudflare cloudflare:402 network implementation
specs/extensions/http-message-signatures.md	Extension for RFC 9421 authentication
specs/extensions/terms.md	Extension for communicating usage terms

Protocol Flow

1. Client → Server: GET /resource
2. Server → Client: 402 + PAYMENT-REQUIRED (accepts[], extensions)
3. Client: Signs request with HTTP Message Signatures (RFC 9421)
4. Client → Server: GET /resource + Signature headers
5. Server: Verifies signature agent, validates payment commitment
6. Server → Client: 200 + PAYMENT-RESPONSE + content
7. Network: Settles via off-chain rails (daily/weekly batching)

Tests

None, this is a spec-only PR (no SDK implementation).

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• SKIPPED added a changelog fragment for user-facing changes (docs-only changes can skip)

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[vercel]

@CameronWhiteside is attempting to deploy a commit to the Coinbase Team on Vercel.

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