Function metamer_kernel_basis 

pub fn metamer_kernel_basis(
    out_basis: &mut [f32],
) -> Result<usize, MetamerError>

Compute a basis for the kernel of the CMF projection (metameric-black SPDs).

The kernel has dimension SPD_SAMPLES - 3 = 38. We compute it via Gram-Schmidt orthogonalisation against the three CMF rows.

out_basis needs (SPD_SAMPLES - 3) * SPD_SAMPLES entries (row-major). Returns the number of basis vectors written.