Function is_heartbeat_late 

pub fn is_heartbeat_late(
    last_heartbeat_at: Option<DateTime<Utc>>,
    expected_interval_secs: i32,
    paused_at: Option<DateTime<Utc>>,
    now: DateTime<Utc>,
) -> bool

Whether a phase’s last heartbeat is older than PHASE_HEARTBEAT_INTERVAL_MULTIPLIER of its intervals. A paused phase is never late.