/**
 * Sends periodic keepalive heartbeats from a remote worker to the server to
 * extend the task visibility timeout while activity execution is in progress.
 *
 * Construct with a `sendHeartbeat` callback (typically the worker's WebSocket
 * send function) and an optional interval in milliseconds (default: 10 000).
 * Call `start()` when a task is acquired and `stop()` when it completes.
 *
 * @example
 * ```ts
 * import { HeartbeatManager } from '@lostgradient/weft';
 *
 * let heartbeatFn = (details?: unknown) => {
 *   console.log('heartbeat', details);
 * };
 *
 * const manager = new HeartbeatManager(heartbeatFn, 5_000);
 * manager.start();
 * // ... execute long-running activity ...
 * manager.beat({ progress: 0.5 }); // one-off heartbeat with details
 * manager.stop();
 * ```
 */
export declare class HeartbeatManager {
    #private;
    constructor(sendHeartbeat: (details?: unknown) => void, intervalMs?: number);
    /** Start sending periodic heartbeats. */
    start(): void;
    /** Stop sending heartbeats. */
    stop(): void;
    /** Send a one-off heartbeat with optional details. */
    beat(details?: unknown): void;
    get isRunning(): boolean;
}