/**
 * Stringify an arbitrary error value for log output without producing
 * `[object Object]`.
 *
 * The `err?.message ?? String(err)` idiom that used to appear in several
 * places quietly degrades to `[object Object]` when `err` is a plain object
 * without a `.message` property — which Effect.ts and some SDK errors are.
 * `formatErr` falls back to `JSON.stringify` so log output always carries
 * the actual payload.
 */
export declare function formatErr(err: unknown): string;
