Function on_first_data 

pub fn on_first_data(
    core: &Core,
    binding: &Arc<dyn ProducerBinding>,
    source: NodeId,
    fn_id: FnId,
) -> NodeId

One-shot side-effect tap. Calls invoke_tap_fn(fn_id, handle) on the first DATA only, then becomes a pure passthrough. All messages are forwarded unchanged.