Function integrate_euler_3d 

pub fn integrate_euler_3d(
    x: f64,
    y: f64,
    z: f64,
    vx: f64,
    vy: f64,
    vz: f64,
    fx: f64,
    fy: f64,
    fz: f64,
    dt: f64,
    params: &SocialForceParams,
) -> (f64, f64, f64, f64, f64, f64)

Euler integration with speed clamping in 3D.

Takes an acceleration (m/s²), updates velocity, clamps speed, advances position.

Returns (new_x, new_y, new_z, new_vx, new_vy, new_vz).