Function spawn_rebalance_controller 

pub fn spawn_rebalance_controller(
    db: Arc<LaminarDB>,
    controller: Arc<ClusterController>,
    store: Arc<AssignmentSnapshotStore>,
    registry: Arc<VnodeRegistry>,
    shutdown: Arc<Notify>,
    config: RebalanceConfig,
) -> JoinHandle<()>

Spawn the leader-gated rebalance controller. Runs on every node; leadership is re-checked after the debounce.