Gas oracle / gas price API / EIP-1559 fee estimator / Base + Ethereum + Optimism + Arbitrum + Polygon gas tracker / etherscan-gas alternative / pre-tx cost estimator / USD gas-cost calculator / safe-low + standard + fast tier feed. Reads the chain's latest block, derives the EIP-1559 base fee, suggests priority fees for safe-low / standard / fast tiers, computes the total recommended maxFeePerGas, and converts cost to USD for common tx types (transfer, ERC-20 transfer, Uniswap swap, NFT mint).