Function polygon_offset_3d 

pub fn polygon_offset_3d(
    vertices: &[[f64; 3]],
    distance: f64,
    plane_normal: [f64; 3],
) -> Vec<[f64; 3]>

Offset a 3-D polygon (planar) by distance along edge normals.

The polygon lives in [f64; 3] space and is assumed planar. This projects to 2-D, offsets, and lifts back.