/**
 * Format a plain datetime range using the Temporal Intl.DateTimeFormat formatRange API.
 *
 * - Plain counterpart of `formatZonedRange` — same parameter order and option shape.
 * - Uses Temporal.PlainDateTime.from for both endpoints; no timezone is involved.
 * - Locale elides shared fields between `start` and `end` (e.g. same day/month/year).
 * - Returns "" for invalid input on either endpoint.
 *
 * @param start ISO PlainDateTime string (range start)
 * @param end ISO PlainDateTime string (range end)
 * @param locale optional locale tag
 * @param options optional Intl.DateTimeFormatOptions
 * @returns localized range string or "" when invalid
 *
 * @example formatDateTimeRange("2024-02-03T09:00:00", "2024-02-03T17:00:00", "en-US", { dateStyle: "long", timeStyle: "short" }) // "February 3, 2024, 9:00 AM – 5:00 PM"
 * @example formatDateTimeRange("2024-02-03T09:00:00", "2024-02-10T17:00:00", "en-US", { dateStyle: "long", timeStyle: "short" }) // "February 3, 2024 at 9:00 AM – February 10, 2024 at 5:00 PM"
 * @example formatDateTimeRange("invalid", "2024-02-03T17:00:00", "en-US") // "" (invalid input)
 */
export declare function formatDateTimeRange(start: string, end: string, locale?: string, options?: Intl.DateTimeFormatOptions): string;