Function reconstruct_node_with_length 

pub fn reconstruct_node_with_length(
    approximation: &[f64],
    detail: &[f64],
    wavelet: &dyn Wavelet,
    boundary_mode: BoundaryMode,
    parent_len: usize,
) -> Result<Vec<f64>>

Reconstruct a parent node from its two children and the exact parent length.

parent_len is the length of the parent band (i.e. the input length that the forward step at this level consumed). This avoids the parity ambiguity in DWT output lengths, where the obvious child.len() * 2 is wrong for odd-N Periodic and for non-Haar non-Periodic inputs.