Function solve_poisson_fft 

fn solve_poisson_fft(
    f: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
) -> Vec<f64>

Solve Poisson equation using FFT (periodic boundary conditions)

Solves: ∇²u = f In Fourier domain: λ * û = f̂, where λ are eigenvalues of discrete Laplacian