Function send_interval 

pub fn send_interval<A, M>(
    period: Duration,
    ctx: Context<A>,
    msg: M,
) -> TimerHandle
where
    A: Actor + Handler<M>,
    M: Message + Clone,

Send a message to an actor repeatedly at a fixed interval.

The message type must implement Clone since a copy is sent on each tick. For #[protocol]-generated messages, unit structs (no fields) derive Clone automatically. For structs with fields, implement Clone manually on the generated message struct (e.g., impl Clone for my_protocol::MyMessage { .. }).