Function build_grid 

pub fn build_grid(
    x: &[f32],
    y: &[f32],
    z: &[f32],
    origin: [f32; 3],
    dims: [u32; 3],
    cell_size: f32,
) -> (Vec<u32>, Vec<u32>)

Counting-sort points into grid cells, returning sorted indices and CSR offsets.