Function gpu_particle_integrate_verlet 

pub fn gpu_particle_integrate_verlet(
    particles: &mut Vec<SphParticle>,
    dt: f64,
    _dt_prev: f64,
)

GPU Verlet integration using the previous time-step dt_prev.

x_new = x + vdt + 0.5a*dt^2 (Störmer–Verlet, velocity-explicit variant).