feat(evm): add optional ERC-8021 Builder Code attribution to settlement transactions

[CECILIA-MULANDI]

Description

Adds optional ERC-8021 (Builder Code) attribution to facilitator settlement transactions.

When builderCode is configured on ExactEvmScheme or UptoEvmScheme, an ERC-8021 trailing suffix is appended to settlement calldata. Smart contracts ignore trailing calldata beyond ABI-decoded arguments, so this is non-breaking. Without builderCode set, all paths fall through to the existing writeContract behavior — zero change.

Covers all settlement paths: Permit2 direct, settleWithPermit (EIP-2612), ERC-20 approval, and EIP-3009 transferWithAuthorization.

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Usage

new ExactEvmScheme(signer, { builderCode: "myapp" })
new UptoEvmScheme(signer, { builderCode: "myapp" })

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What Changed

• New erc8021.ts utility with buildErc8021Suffix() and writeContractWithBuilderCode() helper
• Added builderCode?: string to ExactEvmSchemeConfig, UptoEvmSchemeConfig, Permit2FacilitatorConfig, and EIP3009FacilitatorConfig
• Modified all settle functions to use writeContractWithBuilderCode (or append suffix to raw calldata for extension signer paths)
• Exported buildErc8021Suffix from @x402/evm

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Tests

• pnpm build --filter @x402/evm passes cleanly
• pnpm test --filter @x402/evm — 339/339 tests pass
• pnpm lint --filter @x402/evm — 0 errors
• Existing tests unaffected (no behavioral change without builderCode)

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Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• I added a changelog fragment for user-facing changes