chore: migrate x402-go from mark3labs v0.13.0 to coinbase/x402/go v2

[bussyjd]

Context

The stack pins github.com/mark3labs/x402-go v0.13.0 (legacy v1 SDK). The official Go SDK inside coinbase/x402 is now at v2.7.0 with a complete rewrite. The v1 API surface has been moved to /legacy subfolders upstream and will eventually go stale.

Additionally, the repo is being renamed from coinbase/x402 → x402-foundation/x402 (#1912), which will eventually break the Go module path.

Current v1 Usage (6 production files + tests)

File	v1 Imports	Key Usage
internal/x402/config.go	x402lib (root)	ChainConfig constants (BaseMainnet, BaseSepolia, PolygonMainnet, etc.), ResolveChain()
internal/x402/verifier.go	x402lib (root), x402http	NewUSDCPaymentRequirement(), NewX402Middleware() (ForwardAuth)
internal/x402/buyer/signer.go	x402 (root)	Implements x402.Signer interface (7 methods), uses PaymentRequirement, PaymentPayload, EVMPayload, EVMAuthorization, TokenConfig, ErrNoValidSigner
internal/x402/buyer/proxy.go	x402 (root), encoding	PaymentSelector, PaymentCallback, PaymentEvent, NewDefaultPaymentSelector(), NewPaymentError(), encoding.EncodePayment(), encoding.DecodeSettlement()
internal/inference/gateway.go	x402 (root), x402http	ChainConfig, NewUSDCPaymentRequirement(), NewX402Middleware()
cmd/obol/sell.go	x402 (root)	Chain validation references

v2 Changes Relevant to the Stack

High Priority

• Go SDK v2 full rewrite — new architecture: mechanisms (EVM/SVM), extensions, hooks, builders. Module path: github.com/coinbase/x402/go
• validAfter timing — EIP-3009 buffer increased 30s → 10min, fixes edge-case payment rejections
• HTTPFacilitatorClient 308 redirect fix — broken redirects could cause silent verification failures
• ERC-1271 / ERC-6492 smart contract wallet signatures — needed for EIP-7702 / Safe wallets

Medium Priority

• Payment-identifier extension — idempotent payments, prevents duplicate settlement
• Extensions field on VerifyResponse/SettleResponse — new metadata from facilitator
• New chains: Polygon (137), Arbitrum One (42161), Arbitrum Sepolia (421614), MegaETH (4326), Stable (988/2201), Monad (143)

Low Priority (future)

• Permit2 / Upto scheme support
• MCP transport integration
• Bazaar dynamic routing

Migration Surface

Types

• ChainConfig → evm.NetworkConfig (CAIP-2 format: "eip155:8453")
• PaymentRequirement → types.PaymentRequirements (MaxAmountRequired → Amount, Resource → ResourceInfo)
• PaymentPayload → types.PaymentPayload (scheme/network move inside Accepted)
• EVMPayload / EVMAuthorization → evm.ExactEIP3009Payload / ExactEIP3009Authorization

Interfaces

• x402.Signer (7 methods) → SchemeNetworkClient + ClientEvmSigner
• x402.PaymentSelector → built into x402Client.SelectPaymentRequirements()
• x402http.NewX402Middleware() → http.NewServer() with RoutesConfig

Encoding

• encoding.EncodePayment() / encoding.DecodeSettlement() — need v2 equivalents or inline

Approach

1. Phase 1: Add github.com/coinbase/x402/go v2 to go.mod, coexist with v1 via aliased imports
2. Phase 2: Migrate internal/x402/config.go (chain constants → CAIP-2)
3. Phase 3: Migrate verifier (NewX402Middleware → http.NewServer)
4. Phase 4: Migrate buyer signer + proxy (Signer interface, encoding, PaymentSelector)
5. Phase 5: Migrate standalone inference gateway
6. Phase 6: Remove mark3labs/x402-go from go.mod

Each phase should be a separate commit, all in one PR.