Function prepare_fansi_spectral 

pub fn prepare_fansi_spectral(
    grid: &Grid,
    bdir: (f64, f64, f64),
) -> (Fft3dWorkspace, Vec<f64>, Vec<f64>)

Pre-compute spectral operators shared by the FANSI-family nonlinear solvers (NDI, nlTV, nlTGV, L1-QSM, WH-QSM, HD-QSM).

Unlike prepare_admm_spectral, this does not bake the data term into an RHS, because those solvers carry a separately-updated data auxiliary variable. It returns the reusable primitives instead:

• Fft3dWorkspace — reusable FFT plans (unnormalized forward fft3d, normalized inverse ifft3d, matching MATLAB fftn/ifftn).
• k_kernel — the real k-space dipole kernel D(k) (same convention as the rest of the crate: continuous kernel, includes voxel-size scaling, DC = 0).
• ee2 — the real spectral Laplacian EE2(k) = |E1|² + |E2|² + |E3|², i.e. the frequency response of bdiv ∘ fgrad. Use as the regularization term in the x-subproblem denominator so it stays consistent with real-space fgrad_inplace/bdiv_inplace (which share the same voxel-size scaling).

The x-subproblem denominator for these solvers is therefore mu2 * k_kernel² + mu * ee2 (all real), matching FANSI’s mu2*abs(Kernel).^2 + mu*EE2.