Function rescue 

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

Error recovery operator. On ERROR, calls binding.invoke_rescue_fn(fn_id, error_handle):

• Ok(recovered_handle) — emit DATA with the recovered value.
• Err(()) — forward the original ERROR unchanged.

DATA and COMPLETE pass through unchanged.