/**
 * Computes a 2-character hex shard key for a given ID.
 *
 * For hex SHAs (exactly 40 or 64 hex chars), uses the first two characters (lowercased).
 * For all other strings, computes FNV-1a hash over UTF-8 bytes and takes the low byte.
 *
 * Returns '00' for null, undefined, or non-string inputs (graceful fallback).
 */
export default function computeShardKey(id: string): string;
