/**
 * Starts a recurring heartbeat loop driven by the active Activity's
 * `heartbeatTimeoutMs`. The loop fires immediately, then re-schedules
 * itself at `heartbeatTimeoutMs / 2` to keep well clear of the timeout.
 *
 * Returns a `stop` function that cancels the loop. If the Activity has no
 * `heartbeatTimeoutMs` configured, no heartbeats are sent and `stop` is a no-op.
 *
 * Heartbeat errors are swallowed: the Activity may already be cancelled, in
 * which case the next `heartbeat()` call would throw and we don't want that
 * to mask the Activity's primary work.
 */
export declare function startAdaptiveHeartbeat(): () => void;