Function shell 

pub fn shell(
    topo: &mut Topology,
    solid: SolidId,
    thickness: f64,
    open_faces: &[FaceId],
) -> Result<SolidId, OperationsError>

Create a hollow shell from a solid by offsetting faces inward.

Each face is offset inward by thickness along its outward normal. Supports planar, NURBS, and analytic surface faces. If open_faces is non-empty, those faces are removed from both the outer and inner shells, creating openings.

Errors

Returns an error if:

• thickness is non-positive
• Any face in open_faces is not part of the solid
• Face offset fails (e.g., negative radius for curved surfaces)
• The resulting shell is degenerate