fix: restore compute_volume function signature in 3d_print

examples/08_3d_print/main.cpp

@@ -24,6 +24,7 @@ using namespace vde::foundation;
 // ── helpers ──
 
 /// Compute approximate volume via divergence theorem (loop over faces)
+static double compute_volume(const HalfedgeMesh& m) {
     double vol = 0.0;
     for (size_t fi = 0; fi < m.num_faces(); ++fi) {
         auto verts = m.face_vertices(fi);