/**
 * test_watchdog_heartbeat — Story 6.6 regression suite.
 *
 * Three scenarios:
 *   1. start() emits immediately and writes a fresh heartbeat file
 *   2. periodic emission updates `ts` after intervalMs elapses
 *   3. stop() halts emission; subsequent file mtime stays frozen
 *
 * Prints TEST_WATCHDOG_HEARTBEAT_OK on full pass.
 */
export {};
