feat(v3.8): G2 curvature + continuity implementation

- surface_curvature: mean (H) and Gaussian (K) curvature via
  first/second fundamental forms with analytic first derivatives
  and finite-difference second derivatives
- is_g2_continuous: G0/G1/G2 continuity check along shared boundary
  with proper parameter range extraction and reverse-parameter
  matching on opposite edges
- extend_surface: tangent-aligned surface extension along any edge
  (umin/umax/vmin/vmax) with knot vector propagation

include/vde/curves/nurbs_operations.h

@@ -2,6 +2,7 @@
 #include "vde/curves/nurbs_surface.h"
 #include "vde/curves/nurbs_curve.h"
 #include "vde/core/point.h"
+#include <utility>
 #include <vector>
 
 namespace vde::curves {