fix: SDF test files — core namespace qualification

tests/sdf/test_sdf_to_mesh.cpp

@@ -2,7 +2,7 @@
 #include <gtest/gtest.h>
 
 using namespace vde::sdf;
-using core::Point3D;
+using vde::core::Point3D;
 
 TEST(SdfToMesh, SphereProducesValidMesh) {
     auto sphere = SdfNode::sphere(2.0);

tests/sdf/test_sdf_tree.cpp

@@ -3,8 +3,8 @@
 #include <cmath>
 
 using namespace vde::sdf;
-using core::Point3D;
+using vde::core::Point3D;
-using core::Vector3D;
+using vde::core::Vector3D;
 
 // ── Null/empty tree ──────────────────────────────────────────────