Function wcsph_euler_step 

pub fn wcsph_euler_step(
    positions: &[[f64; 3]],
    velocities: &[[f64; 3]],
    forces: &[[f64; 3]],
    masses: &[f64],
    dt: f64,
) -> (Vec<[f64; 3]>, Vec<[f64; 3]>)

Apply one explicit WCSPH Euler step.

Updates positions and velocities using the computed forces. Returns (new_positions, new_velocities).