feat(specs): Add subscribe scheme specification for recurring payments

[asghaier76]

Introduces a new payment scheme to complement the existing scheme,
enabling subscription-based recurring payments for internet resources.

Key features:

• Subscription lifecycle: discovery, subscribe, access with proof, renew/cancel
• Pre-authorized renewals via EIP-3009 or Permit2 signatures
• Subscription proof header for efficient access verification without per-request payments
• Grace period handling for failed renewals
• Support for multiple billing cycles (daily, weekly, monthly, annual)
• AI agent optimization for automatic payment scheme selection

Includes:

• specs/schemes/subscribe/scheme_subscribe.md - Scheme overview
• specs/schemes/subscribe/scheme_subscribe_evm.md - EVM-specific implementation with reference smart contracts (x402SubscriptionRegistry, x402SubscriptionProxy)

References: #512

Description

Tests

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
• I 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]

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