docs: Circle & Arc integration exploration

[Huygon764]

Summary

Exploration (not production) of integrating Circle products into PolyPay and the feasibility of deploying on Circle's Arc L1. Adds one report at docs/research/circle-arc-exploration.md. Every claim is sourced; no code changes.

Key findings

Arc deploy — demo-only, not production.

• Arc has no live mainnet (public testnet only; mainnet targeted "summer 2026", no exact date) and no zkVerify aggregation contract, which PolyPay's on-chain signer-privacy depends on.
• A private Arc testnet demo is still possible two ways: (a) Horizen Labs deploys zkVerify on Arc, or (b) PolyPay ships a standalone UltraHonk Verifier.sol and verifies proofs directly on-chain — Arc's Osaka EVM baseline keeps the required BN254/modexp precompiles.
• "No Kurier on Arc" is not the real blocker — Kurier is an off-chain, chain-agnostic API. The hard dependency is the on-chain zkVerify contract.

Circle products.

• CCTP is the best fit — fund a multisig cross-chain with native USDC, scoped to Base/Arbitrum (Horizen is not a CCTP chain).
• Gateway is a larger treasury-UX follow-up (also Base/Arbitrum, not Horizen).
• Ruled out: EURC (live only on Base among PolyPay's chains), Paymaster (users already gasless), Wallets (conflicts with ZK identity).