spurious lattice type warnings on loops through `lattice_fold`

[jhellerstein]

See the test case test_lattice_fold_loop (from hydroflow/tests/surface_lattice_fold.rs and pasted below.) It generates the following warning:

warning: Input to `union()` will be downcast to `None` to match other inputs.
  --> hydroflow/tests/surface_lattice_fold.rs:42:16
   |
42 |             -> folder;
   |                ^^^^^^

• As a side note, the text of the warning is unhelpful. Wasn't clear to me that it was related to lattices at all.

fn test_lattice_fold_loop() {
    let (output_send, output_recv) = hydroflow::util::unbounded_channel::<u8>();
    let mut df = hydroflow_syntax! {
        start = source_iter_delta([1])
            -> map(Max::<u8>::new)
            -> folder;
        folder = union()
            -> lattice_fold::<'static>(|| Max::<u8>::new(0))
            -> cast(Some(Delta))
            -> map(|x| Max::<u8>::new(x.into_reveal() + 1))
            -> filter(|x| !x.is_top())
            -> tee();
        folder -> cast(Some(Delta)) -> folder;
        folder
            -> for_each(|v: Max<u8>| output_send.send(*v.as_reveal_ref()).unwrap());
    };
    df.run_tick();
    assert_eq!(
        &(2..=254).collect::<Vec<u8>>(),
        &*collect_ready::<Vec<_>, _>(output_recv)
    );
}