Engine: 实现 Components 和 Scene 模块，包含完整单元测试

新增 Components 模块:
- Component 基类 (生命周期、启用状态管理)
- TransformComponent (本地/世界空间变换、矩阵缓存、父子层级)
- GameObject (组件管理、父子层级、激活状态、静态查找)

新增 Scene 模块:
- Scene (场景管理、对象创建销毁、查找、生命周期)
- SceneManager (单例模式、多场景管理、场景切换)

新增测试:
- test_component.cpp (12 个测试)
- test_transform_component.cpp (35 个测试)
- test_game_object.cpp (26 个测试)
- test_scene.cpp (20 个测试)
- test_scene_manager.cpp (17 个测试)

所有测试均已编译通过。

tests/Components/test_component.cpp

@@ -0,0 +1,110 @@
+#include <gtest/gtest.h>
+#include <XCEngine/Components/Component.h>
+#include <XCEngine/Components/GameObject.h>
+
+using namespace XCEngine::Components;
+
+namespace {
+
+class TestComponent : public Component {
+public:
+    TestComponent() = default;
+    explicit TestComponent(const std::string& name) : m_customName(name) {}
+
+    std::string GetName() const override { 
+        return m_customName.empty() ? "TestComponent" : m_customName; 
+    }
+
+    bool m_awakeCalled = false;
+    bool m_startCalled = false;
+    bool m_updateCalled = false;
+    bool m_onEnableCalled = false;
+    bool m_onDisableCalled = false;
+    bool m_onDestroyCalled = false;
+
+    void Awake() override { m_awakeCalled = true; }
+    void Start() override { m_startCalled = true; }
+    void Update(float deltaTime) override { m_updateCalled = true; }
+    void OnEnable() override { m_onEnableCalled = true; }
+    void OnDisable() override { m_onDisableCalled = true; }
+    void OnDestroy() override { m_onDestroyCalled = true; }
+
+private:
+    std::string m_customName;
+};
+
+TEST(Component_Test, DefaultConstructor) {
+    TestComponent comp;
+    EXPECT_EQ(comp.GetName(), "TestComponent");
+}
+
+TEST(Component_Test, GetGameObject_ReturnsNullptr_WhenNotAttached) {
+    TestComponent comp;
+    EXPECT_EQ(comp.GetGameObject(), nullptr);
+}
+
+TEST(Component_Test, IsEnabled_DefaultTrue) {
+    TestComponent comp;
+    EXPECT_TRUE(comp.IsEnabled());
+}
+
+TEST(Component_Test, SetEnabled_TrueToFalse) {
+    TestComponent comp;
+    EXPECT_TRUE(comp.IsEnabled());
+    comp.SetEnabled(false);
+    EXPECT_FALSE(comp.IsEnabled());
+}
+
+TEST(Component_Test, SetEnabled_FalseToTrue) {
+    TestComponent comp;
+    comp.SetEnabled(false);
+    EXPECT_FALSE(comp.IsEnabled());
+    comp.SetEnabled(true);
+    EXPECT_TRUE(comp.IsEnabled());
+}
+
+TEST(Component_Test, Lifecycle_AwakeCalled) {
+    TestComponent comp;
+    comp.Awake();
+    EXPECT_TRUE(comp.m_awakeCalled);
+}
+
+TEST(Component_Test, Lifecycle_StartCalled) {
+    TestComponent comp;
+    comp.Start();
+    EXPECT_TRUE(comp.m_startCalled);
+}
+
+TEST(Component_Test, Lifecycle_UpdateCalled) {
+    TestComponent comp;
+    comp.Update(0.016f);
+    EXPECT_TRUE(comp.m_updateCalled);
+}
+
+TEST(Component_Test, Lifecycle_OnEnableCalled_WhenEnabling) {
+    TestComponent comp;
+    comp.SetEnabled(false);
+    comp.SetEnabled(true);
+    EXPECT_TRUE(comp.m_onEnableCalled);
+}
+
+TEST(Component_Test, Lifecycle_OnDisableCalled_WhenDisabling) {
+    TestComponent comp;
+    comp.SetEnabled(false);
+    EXPECT_TRUE(comp.m_onDisableCalled);
+}
+
+TEST(Component_Test, Lifecycle_OnDestroyCalled) {
+    TestComponent comp;
+    comp.OnDestroy();
+    EXPECT_TRUE(comp.m_onDestroyCalled);
+}
+
+TEST(Component_Test, SetEnabled_NoCallback_WhenStateUnchanged) {
+    TestComponent comp;
+    comp.SetEnabled(true);
+    EXPECT_FALSE(comp.m_onEnableCalled);
+    EXPECT_FALSE(comp.m_onDisableCalled);
+}
+
+} // namespace