/**
 * Recursively merge source objects into target.
 * Arrays and non-plain objects are replaced, not merged.
 *
 * @param target - The target object
 * @param sources - The source objects
 * @returns New merged object
 *
 * @example
 * mergeFast({ a: { b: 1 } }, { a: { c: 2 } })
 * // { a: { b: 1, c: 2 } }
 */
export declare function mergeFast<T extends object>(...sources: T[]): T;
/**
 * Like mergeFast, but with a customizer function for fine-grained control.
 *
 * @param target - The target object
 * @param source - The source object
 * @param customizer - Function to customize merging. Return undefined to use default behavior.
 * @returns New merged object
 *
 * @example
 * mergeWithFast({ a: [1] }, { a: [2] }, (target, source) => {
 *   if (Array.isArray(target) && Array.isArray(source)) {
 *     return [...target, ...source];
 *   }
 * })
 * // { a: [1, 2] }
 */
export declare function mergeWithFast<T extends object>(target: T, source: T, customizer: (targetValue: unknown, sourceValue: unknown, key: string) => unknown): T;
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