add robinhood-mainnet (4663) support

Robinhood chain (chainId 4663) is a standard ETH-native chain: reference token is WETH, priced
via the WETH/USDG pool. No pricing override (unlike Arc). USDG ("Global Dollar", 6 dec) is the
USD stablecoin.

- config/robinhood-mainnet: V3 factory 0x1f7d7550... @ startBlock 8930; REFERENCE_TOKEN = WETH
  (0x0Bd7D308...), STABLE_COINS = [USDG], WHITELIST = [WETH, USDG]
- register robinhood-mainnet in the NETWORK enum

STABLE_TOKEN_POOL is a zero-address placeholder until the WETH/USDG v3 pool is created + seeded;
until then USD metrics read 0 (indexing works). Addresses per Uniswap/contracts deployments/4663.md.