/**
 * EIP-55 checksum an Ethereum address.
 *
 * If the input contains mixed case, it is validated as an existing checksum —
 * an incorrect checksum throws (matching ethers.getAddress behavior).
 * All-lowercase and all-uppercase inputs are checksummed without validation.
 *
 * @param address The address to checksum (must be a valid 0x-prefixed hex address).
 * @returns The checksummed address.
 * @throws Error if the address is not a valid Ethereum address or has an invalid checksum.
 */
export declare function checksumAddress(address: string): string;