/**
 * Header merging: defaults < client-config < per-call < auth.
 *
 * Case-insensitive: later layers override earlier even if the casing of the
 * header name differs. The returned object preserves the casing of the
 * highest-priority layer's key.
 */
export declare function mergeHeaders(...layers: Array<Record<string, string> | undefined>): Record<string, string>;