Add owner-gated Base Builder attribution

[omslice]

What changed

• adds owner-gated ERC-8021 Builder Code attribution for direct Privy transactions and wagmi contract writes
• keeps attribution disabled when no real Base.dev Builder Code is configured
• documents Base.dev registration, testnet configuration, and authentic-transaction verification
• adds focused fail-closed unit coverage and the official ox/erc8021 encoder

Why

Authentic Builder Code attribution gives OpenEscrow verifiable Base.dev usage evidence for ecosystem discovery and Builder Rewards without changing the contracts or inventing activity. The code remains inert until the owner registers the app and supplies the issued code.

Impact and boundaries

No contract deployment, site deployment, demo change, wallet connection, or transaction is included. The existing testnet behavior is unchanged when VITE_BASE_BUILDER_CODE is unset.

Validation

• npm run release:check:frontend
• npm run test:client-logic (316 passing)
• production build with attribution unset
• production build with a representative test Builder Code
• reviewer evidence: 53 files, 0 working-tree changes, 0 blockers
• production dependency audit: 0 high, 0 critical