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/CMakeLists.txt

@@ -0,0 +1,50 @@
+cmake_minimum_required(VERSION 3.15)
+project(XCEngineTests)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+# ============================================================
+# Test Configuration
+# ============================================================
+
+option(ENABLE_COVERAGE "Enable code coverage" OFF)
+option(ENABLE_BENCHMARK "Enable benchmark tests" OFF)
+
+# ============================================================
+# Dependencies
+# ============================================================
+
+include(FetchContent)
+FetchContent_Declare(
+    googletest
+    GIT_REPOSITORY https://gitee.com/mirrors/googletest.git
+    GIT_TAG v1.14.0
+)
+
+set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(googletest)
+
+enable_testing()
+
+# ============================================================
+# Engine Library
+# ============================================================
+
+add_subdirectory(../engine engine)
+
+# ============================================================
+# Test Subdirectories
+# ============================================================
+
+add_subdirectory(math)
+
+# ============================================================
+# Test Summary
+# ============================================================
+
+add_custom_target(print_tests
+    COMMAND ${CMAKE_COMMAND} -E echo "===== XCEngine Test Suite ====="
+    COMMAND ${CMAKE_CTEST_COMMAND} -N
+    COMMENT "Available tests:"
+)