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/Scene/test_scene.cpp

@@ -0,0 +1,217 @@
+#include <gtest/gtest.h>
+#include <XCEngine/Scene/Scene.h>
+#include <XCEngine/Components/GameObject.h>
+#include <XCEngine/Components/TransformComponent.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_updateCalled = false;
+
+    void Awake() override { m_awakeCalled = true; }
+    void Update(float deltaTime) override { m_updateCalled = true; }
+
+private:
+    std::string m_customName;
+};
+
+class SceneTest : public ::testing::Test {
+protected:
+    void SetUp() override {
+        testScene = std::make_unique<Scene>("TestScene");
+    }
+
+    std::unique_ptr<Scene> testScene;
+};
+
+TEST(Scene_Test, DefaultConstructor_DefaultName) {
+    Scene s;
+    
+    EXPECT_EQ(s.GetName(), "Untitled");
+}
+
+TEST(Scene_Test, NamedConstructor) {
+    Scene s("MyScene");
+    
+    EXPECT_EQ(s.GetName(), "MyScene");
+}
+
+TEST_F(SceneTest, CreateGameObject_Simple) {
+    GameObject* go = testScene->CreateGameObject("TestObject");
+    
+    EXPECT_NE(go, nullptr);
+    EXPECT_EQ(go->GetName(), "TestObject");
+}
+
+TEST_F(SceneTest, CreateGameObject_WithParent) {
+    GameObject* parent = testScene->CreateGameObject("Parent");
+    GameObject* child = testScene->CreateGameObject("Child", parent);
+    
+    EXPECT_NE(child, nullptr);
+    EXPECT_EQ(child->GetParent(), parent);
+}
+
+TEST_F(SceneTest, CreateGameObject_AssignsID) {
+    GameObject* go1 = testScene->CreateGameObject("Object1");
+    GameObject* go2 = testScene->CreateGameObject("Object2");
+    
+    EXPECT_NE(go1->GetID(), GameObject::INVALID_ID);
+    EXPECT_NE(go2->GetID(), GameObject::INVALID_ID);
+    EXPECT_NE(go1->GetID(), go2->GetID());
+}
+
+TEST_F(SceneTest, CreateGameObject_AddsToRoot) {
+    GameObject* go = testScene->CreateGameObject("RootObject");
+    
+    auto roots = testScene->GetRootGameObjects();
+    
+    EXPECT_EQ(roots.size(), 1u);
+    EXPECT_EQ(roots[0], go);
+}
+
+TEST_F(SceneTest, DestroyGameObject_Simple) {
+    GameObject* go = testScene->CreateGameObject("TestObject");
+    
+    testScene->DestroyGameObject(go);
+    
+    EXPECT_EQ(testScene->Find("TestObject"), nullptr);
+}
+
+TEST_F(SceneTest, DestroyGameObject_WithChildren) {
+    GameObject* parent = testScene->CreateGameObject("Parent");
+    testScene->CreateGameObject("Child", parent);
+    
+    testScene->DestroyGameObject(parent);
+    
+    EXPECT_EQ(testScene->Find("Parent"), nullptr);
+    EXPECT_EQ(testScene->Find("Child"), nullptr);
+}
+
+TEST_F(SceneTest, Find_Exists) {
+    testScene->CreateGameObject("FindMe");
+    
+    GameObject* found = testScene->Find("FindMe");
+    
+    EXPECT_NE(found, nullptr);
+    EXPECT_EQ(found->GetName(), "FindMe");
+}
+
+TEST_F(SceneTest, Find_NotExists) {
+    GameObject* found = testScene->Find("NonExistent");
+    
+    EXPECT_EQ(found, nullptr);
+}
+
+TEST_F(SceneTest, GetRootGameObjects_ReturnsTopLevel) {
+    GameObject* parent = testScene->CreateGameObject("Parent");
+    testScene->CreateGameObject("Child", parent);
+    
+    auto roots = testScene->GetRootGameObjects();
+    
+    EXPECT_EQ(roots.size(), 1u);
+    EXPECT_EQ(roots[0], parent);
+}
+
+TEST(Scene_Test, GetRootGameObjects_EmptyScene) {
+    Scene emptyScene;
+    
+    auto roots = emptyScene.GetRootGameObjects();
+    
+    EXPECT_EQ(roots.size(), 0u);
+}
+
+TEST_F(SceneTest, Update_UpdatesActiveObjects) {
+    GameObject* go = testScene->CreateGameObject("TestObject");
+    TestComponent* comp = go->AddComponent<TestComponent>();
+    
+    testScene->Update(0.016f);
+    
+    EXPECT_TRUE(comp->m_updateCalled);
+}
+
+TEST_F(SceneTest, Update_SkipsInactiveObjects) {
+    GameObject* go = testScene->CreateGameObject("InactiveObject");
+    TestComponent* comp = go->AddComponent<TestComponent>();
+    go->SetActive(false);
+    
+    testScene->Update(0.016f);
+    
+    EXPECT_FALSE(comp->m_updateCalled);
+}
+
+TEST(Scene_Test, IsActive_DefaultTrue) {
+    Scene s;
+    
+    EXPECT_TRUE(s.IsActive());
+}
+
+TEST(Scene_Test, SetActive) {
+    Scene s;
+    s.SetActive(false);
+    
+    EXPECT_FALSE(s.IsActive());
+}
+
+TEST_F(SceneTest, FindObjectOfType) {
+    GameObject* go = testScene->CreateGameObject("TestObject");
+    go->AddComponent<TestComponent>();
+    
+    TestComponent* found = testScene->FindObjectOfType<TestComponent>();
+    
+    EXPECT_NE(found, nullptr);
+}
+
+TEST_F(SceneTest, FindObjectsOfType) {
+    GameObject* go1 = testScene->CreateGameObject("Object1");
+    GameObject* go2 = testScene->CreateGameObject("Object2");
+    go1->AddComponent<TestComponent>();
+    go2->AddComponent<TestComponent>();
+    
+    auto found = testScene->FindObjectsOfType<TestComponent>();
+    
+    EXPECT_EQ(found.size(), 2u);
+}
+
+TEST_F(SceneTest, FindGameObjectWithTag) {
+    GameObject* go = testScene->CreateGameObject("MyTag");
+    
+    GameObject* found = testScene->FindGameObjectWithTag("MyTag");
+    
+    EXPECT_NE(found, nullptr);
+}
+
+TEST_F(SceneTest, OnGameObjectCreated_Event) {
+    bool eventFired = false;
+    testScene->OnGameObjectCreated().Subscribe([&eventFired](GameObject*) {
+        eventFired = true;
+    });
+    
+    testScene->CreateGameObject("TestObject");
+    
+    EXPECT_TRUE(eventFired);
+}
+
+TEST_F(SceneTest, OnGameObjectDestroyed_Event) {
+    bool eventFired = false;
+    testScene->OnGameObjectDestroyed().Subscribe([&eventFired](GameObject*) {
+        eventFired = true;
+    });
+    
+    GameObject* go = testScene->CreateGameObject("TestObject");
+    testScene->DestroyGameObject(go);
+    
+    EXPECT_TRUE(eventFired);
+}
+
+} // namespace