add arc-mainnet (5042) support

Arc (chainId 5042) is Circle's USDC-native L1. Its native gas token is USDC and there is
no wrapped native — the deployment's "WETH9" slot (0x8bceaa40...) is the UnsupportedProtocol
revert stub — so pools pair directly against native USDC and USDC is the reference token.

Because the reference token IS the dollar, getEthPriceInUSD() must return 1 (there's no
wrapped-native/stable pool to read). Chains opt into that by setting
STABLE_TOKEN_POOL = REFERENCE_TOKEN; the guard only fires on that equality, so every other
chain is unaffected.

- pricing.ts: return ONE_BD when STABLE_TOKEN_POOL == REFERENCE_TOKEN
- config/arc-mainnet: V3 factory 0xf0db7b58... @ startBlock 1948019; REFERENCE_TOKEN = USDC
  (0x3600..., 6 dec); STABLE_COINS = [USDC]; WHITELIST = [USDC, EURC, USYC]
- register arc-mainnet in the NETWORK enum

Uniswap contract addresses per Uniswap/contracts deployments/5042.md; startBlock from explorer.arc.io.