Function offset_freeform_face 

pub fn offset_freeform_face(
    solid: &BrepSolid,
    face_id: u64,
    distance: f64,
) -> Result<BrepSolid, String>

Offset a free-form NURBS face whose boundaries have no other incident face. The fitted offset must pass a dense same-parameter residual check. Intrinsic iso boundaries use exact iso-curves; other self-incident trims use crate::image_curve. Rebuilt topology and volume orientation are validated. Neighbour-trimmed faces require general intersection and are refused.