Function vorticity_confinement 

pub fn vorticity_confinement(
    grid: &mut MacGrid,
    vorticity: &[[f64; 3]],
    epsilon: f64,
    dt: f64,
)

Apply vorticity confinement forces to MAC grid velocities.

Vorticity confinement adds a force F = ε * (N × ω) where N is the normalized gradient of |ω|.