Function wall_repulsion_3d 

pub fn wall_repulsion_3d(
    x: f64,
    y: f64,
    z: f64,
    vx: f64,
    vy: f64,
    vz: f64,
    radius: f64,
    wall: &WallSegment3D,
    params: &SocialForceParams,
) -> (f64, f64, f64)

Wall repulsion + physical contact acceleration from a 3D wall segment.

Coefficients are divided by params.mass before returning.

Returns (ax, ay, az) - acceleration (m/s²) on the agent.