Function modwt_into 

pub fn modwt_into<T: SignalType>(
    data: &[T],
    wavelet: &dyn Wavelet,
    wavelet_out: &mut Vec<T>,
    scaling_out: &mut Vec<T>,
) -> Result<()>

Single-level MODWT writing into caller-supplied output buffers.

Both wavelet_out and scaling_out are resized to data.len() and fully overwritten. Lets rolling-window callers reuse the same output Vecs across windows, eliminating the per-window allocation pair.