Function loft_profile_brep_closed 

pub fn loft_profile_brep_closed(
    input_sections: &[Vec<NurbsCurve>],
) -> Result<BrepSolid, String>

§5.8 CLOSED loft: the sections form a RING (the last flows back into the first), producing a capless genus-1 solid. Every interpolation column runs through interpolate_curve_closed, so the ring is C² across the closure — not a welded seam. Topology per skin is the cylinder-wall rectangle turned on its side: u runs along the section curve (open), v through the sections (closed); the section-0 curves themselves serve as the doubled v-seam edges and the corner rings (one closed curve through every section at each section corner) are shared between adjacent skins.