x402: base-sepolia advertises wrong EIP-712 domain name "USD Coin" (on-chain is "USDC")

Start in internal/x402/chains.go at the Base Sepolia EIP3009Name entry and trace its use through the 402 extra fields. Compare the value with the vendored coinbase/x402/go registry in mechanisms/evm/constants.go, then add the regression coverage requested for the resolved EIP-712 domain and 402 extra.name. Done means Base Sepolia advertises "USDC" while other chain values remain correct.

Found during an x402 buyer/seller compatibility audit (paid-MCP obol sell mcp ⇄ external buyer).

Summary

internal/x402/chains.go advertises the EIP-712 domain name "USD Coin" for Base Sepolia (eip155:84532), but the on-chain USDC contract's EIP-712 domain name is "USDC". The obol sell http 402 challenge surfaces this wrong name in extra.name, so any payer that signs TransferWithAuthorization against the advertised domain produces a signature the facilitator rejects.

Evidence

• internal/x402/chains.go:74 — ChainBaseSepolia.EIP3009Name = "USD Coin"; flows into the 402 extra.name (chains.go:237 EIP712Name = c.EIP3009Name, chains.go:326 "name": asset.EIP712Name).
• On-chain ground truth (eth_call name() on 0x036CbD53842c5426634e7929541eC2318f3dCF7e, Base Sepolia): returns "USDC", version "2". Base mainnet 0x833589…2913 returns "USD Coin" — the split is real per-contract.
• The vendored coinbase/x402/go registry agrees with the chain: eip155:84532 → "USDC", eip155:8453 → "USD Coin" (mechanisms/evm/constants.go).

Why it has not surfaced

obol's own buyer (internal/embed/skills/buy-x402/scripts/buy.py) deliberately ignores the seller-advertised extra.name (treats it as a human-readable display string) and signs from its own authoritative per-chain domain table (USDC_EIP712_DOMAIN, base-sepolia → ("USDC","2")). So internal sell http ⇄ buy.py flows on Base Sepolia pass and mask the bug. A spec-conformant external buyer that trusts extra.name will fail every paid request on Base Sepolia.

Fix

• Set ChainBaseSepolia.EIP3009Name = "USDC".
• Add a regression test asserting the resolved Base Sepolia EIP-712 domain name is "USDC" (and that the 402 extra.name matches).
• Audit the other testnet entries (polygon-amoy, avalanche-fuji, arbitrum-sepolia) against each testnet USDC contract's on-chain name() — they currently all hardcode "USD Coin".