Function sample 

pub fn sample(
    core: &Core,
    binding: &Arc<dyn ProducerBinding>,
    source: NodeId,
    notifier: NodeId,
) -> NodeId

Emits the source’s latest DATA each time notifier delivers DATA.

• Source COMPLETE clears stored value; subsequent notifier DATAs no-op.
• Notifier COMPLETE terminates the sample node.
• Either dep ERROR terminates immediately.