Function supervised_spawn 

pub fn supervised_spawn<S: Scanner>(
    scanner: Arc<S>,
    client: Client,
    num_partitions: u16,
    shutdown: Receiver<bool>,
    metrics: Arc<Metrics>,
) -> JoinHandle<()>

Spawn a supervised scanner that restarts on panic.

• Clean shutdown (via watch channel): exits normally.
• Panic: logs, sleeps 5s, increments restart counter, re-spawns.