fix: nurbs_surface ?: 类型不一致

src/curves/nurbs_surface.cpp

@@ -71,7 +71,7 @@ Vector3D NurbsSurface::normal(double u, double v) const {
     Vector3D du = derivative_u(u, v), dv = derivative_v(u, v);
     Vector3D n = du.cross(dv);
     double len = n.norm();
-    return len > 1e-12 ? n / len : Vector3D::UnitZ();
+    return len > 1e-12 ? Vector3D(n / len) : Vector3D::UnitZ();
 }
 
 NurbsSurface NurbsSurface::ruled(const NurbsCurve& c1, const NurbsCurve& c2) {