/**
 * A small deepMerge helper used for config layering
 * (defaults -> per-env overrides -> JSON overrides).
 *
 * Behaviour:
 *   - When a source value is a *plain object*, merge recursively.
 *   - Otherwise (primitive / array / null / undefined), the source replaces
 *     the target.
 *   - Source keys missing on the target are added.
 *
 * Arrays are *replaced* (not concatenated). For configuration purposes
 * (e.g. allowed locales, feature lists), full replacement per environment
 * is almost always what you want.
 *
 * Intentionally written without lodash to keep the runtime dependency
 * surface minimal.
 */
export type DeepPartial<T> = T extends object ? T extends ReadonlyArray<unknown> ? T : {
    [K in keyof T]?: DeepPartial<T[K]>;
} : T;
export declare function deepMerge<T extends object>(target: T, source: DeepPartial<T> | undefined): T;
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