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

tests/math/CMakeLists.txt

@@ -0,0 +1,33 @@
+# ============================================================
+# Math Library Tests
+# ============================================================
+
+set(MATH_TEST_SOURCES
+    test_vector.cpp
+    test_matrix.cpp
+    test_quaternion.cpp
+    test_geometry.cpp
+)
+
+add_executable(xcengine_math_tests ${MATH_TEST_SOURCES})
+
+# Exclude all static runtime libraries to avoid conflicts
+if(MSVC)
+    set_target_properties(xcengine_math_tests PROPERTIES
+        LINK_FLAGS "/NODEFAULTLIB:libcpmt.lib /NODEFAULTLIB:libcmt.lib"
+    )
+endif()
+
+target_link_libraries(xcengine_math_tests
+    PRIVATE
+    XCEngine
+    GTest::gtest
+    GTest::gtest_main
+)
+
+target_include_directories(xcengine_math_tests PRIVATE
+    ${CMAKE_SOURCE_DIR}/engine/include
+    ${CMAKE_SOURCE_DIR}/tests/fixtures
+)
+
+add_test(NAME MathTests COMMAND xcengine_math_tests)