feat: 添加Math库和Google Test测试框架

- 新增Math库: Vector2/3/4, Matrix3/4, Quaternion, Transform, Color等
- 新增测试框架: Google Test (gtest)
- 新增140个单元测试，覆盖Vector, Matrix, Quaternion, Geometry
- VolumeRenderer支持vcpkg的NanoVDB
- 添加TESTING.md测试文档

engine/src/Math/Transform.cpp

@@ -0,0 +1,44 @@
+#include "Math/Transform.h"
+#include "Math/Matrix4.h"
+
+namespace XCEngine {
+namespace Math {
+
+Matrix4 Transform::ToMatrix() const {
+    return Matrix4x4::TRS(position, rotation, scale);
+}
+
+Transform Transform::Inverse() const {
+    Transform result;
+    result.rotation = rotation.Inverse();
+    result.scale = Vector3::One() / scale;
+    result.position = result.rotation * (Vector3::Zero() - position) * result.scale;
+    return result;
+}
+
+Transform Transform::operator*(const Transform& other) const {
+    Transform result;
+    result.position = position + rotation * (scale * other.position);
+    result.rotation = rotation * other.rotation;
+    result.scale = scale * other.scale;
+    return result;
+}
+
+Vector3 Transform::TransformPoint(const Vector3& point) const {
+    return position + rotation * (scale * point);
+}
+
+Vector3 Transform::TransformDirection(const Vector3& direction) const {
+    return rotation * (scale * direction);
+}
+
+Vector3 Transform::InverseTransformPoint(const Vector3& point) const {
+    return (rotation.Inverse() * (point - position)) / scale;
+}
+
+Vector3 Transform::InverseTransformDirection(const Vector3& direction) const {
+    return rotation.Inverse() * direction / scale;
+}
+
+} // namespace Math
+} // namespace XCEngine