refactor: 实体句柄表皮——实体自己管理组件的 API 形态

- 新增 EntityHandle：e.AddComponent<T>() / e.GetComponent<T>() /
  e.Destroy() / e.GetUUID()，转发 entt registry，零存储开销
- Scene::CreateEntity 返回 EntityHandle；Scene 层组件接口保留
  （给渲染器/系统批量遍历用）
- 语义分层：应用层"实体管组件"（句柄表皮），
  数据层组件存储仍集中在 registry（ECS 性能根基）

App/main.cpp

@@ -12,6 +12,7 @@
 #include <Engine/Core/Log.h>
 #include <Engine/Core/Stats.h>
 #include <Engine/ECS/Scene.h>
+#include <Engine/ECS/EntityHandle.h>
 #include <Engine/ECS/Components.h>
 #include <Engine/Assets/AssetManager.h>
 
@@ -42,56 +43,54 @@ protected:
         auto backpack = GetAssets().LoadModel("assets/models/backpack/backpack.obj");
 
         // 相机实体（primary = 当前渲染视角）
-        m_CameraEntity = scene.CreateEntity("MainCamera");
-        auto& camera = scene.AddComponent<CameraComponent>(m_CameraEntity);
+        m_Camera = scene.CreateEntity("MainCamera");
+        auto& camera = m_Camera.AddComponent<CameraComponent>();
         camera.primary = true;
         camera.camera.SetPosition(glm::vec3(0.0f, 0.0f, 7.0f));
 
         // 背包实体 × 3
         for (int i = 0; i < 3; ++i) {
-            Entity e = scene.CreateEntity("Backpack" + std::to_string(i));
+            auto e = scene.CreateEntity("Backpack" + std::to_string(i));
 
-            auto& transform = scene.AddComponent<TransformComponent>(e);
-            transform.world = glm::translate(
+            e.AddComponent<TransformComponent>().world = glm::translate(
                 glm::mat4(1.0f),
                 glm::vec3((i - 1) * 1.5f, 0.0f, 0.0f));
 
-            scene.AddComponent<MeshRendererComponent>(e, MeshRendererComponent{backpack});
+            e.AddComponent<MeshRendererComponent>(MeshRendererComponent{backpack});
 
             m_Spinners.push_back(e);
         }
 
         // 灯光实体
-        Entity keyLight = scene.CreateEntity("KeyLight");
-        scene.AddComponent<LightComponent>(keyLight,
-            LightComponent{glm::vec3(-0.5f, -1.0f, -0.3f),
-                           glm::vec4(1.0f, 0.95f, 0.9f, 1.0f)});
+        scene.CreateEntity("KeyLight")
+            .AddComponent<LightComponent>(
+                LightComponent{glm::vec3(-0.5f, -1.0f, -0.3f),
+                               glm::vec4(1.0f, 0.95f, 0.9f, 1.0f)});
 
-        Entity fillLight = scene.CreateEntity("FillLight");
-        scene.AddComponent<LightComponent>(fillLight,
-            LightComponent{glm::vec3(0.5f, -0.5f, 0.3f),
-                           glm::vec4(0.3f, 0.5f, 1.0f, 0.8f)});
+        scene.CreateEntity("FillLight")
+            .AddComponent<LightComponent>(
+                LightComponent{glm::vec3(0.5f, -0.5f, 0.3f),
+                               glm::vec4(0.3f, 0.5f, 1.0f, 0.8f)});
     }
 
     void OnUpdate(f32 deltaTime) override
     {
-        Scene& scene = GetScene();
         m_Time += deltaTime;
 
         // 相机（WASD/Space/Shift 移动）
-        scene.GetComponent<CameraComponent>(m_CameraEntity)
+        m_Camera.GetComponent<CameraComponent>()
             .camera.Update(GetWindow(), deltaTime);
 
         // 旋转动画：直接写实体组件
         for (size_t i = 0; i < m_Spinners.size(); ++i) {
             f32 angle = m_Time * glm::radians(60.0f) + static_cast<f32>(i) * 0.7f;
 
-            auto& transform = scene.GetComponent<TransformComponent>(m_Spinners[i]);
+            auto& transform = m_Spinners[i].GetComponent<TransformComponent>();
             glm::mat4 t = glm::translate(
                 glm::mat4(1.0f),
                 glm::vec3((static_cast<f32>(i) - 1.0f) * 1.5f, 0.0f, 0.0f));
             glm::mat4 r = glm::rotate(glm::mat4(1.0f), angle, glm::vec3(0.0f, 1.0f, 0.0f));
-            glm::mat4 s = glm::scale(glm::mat4(1.0f), glm::vec3(0.5));
+            glm::mat4 s = glm::scale(glm::mat4(1.0f), glm::vec3(0.5f));
 
             transform.world = t * r * s;
         }
@@ -115,8 +114,8 @@ protected:
     }
 
 private:
-    Entity m_CameraEntity{entt::null};   // 注意：entt::entity{} 是 0 号实体，空句柄要用 entt::null
-    std::vector<Entity> m_Spinners;
+    EntityHandle m_Camera{};                 // Scene::CreateEntity 之前为空
+    std::vector<EntityHandle> m_Spinners;
     f32 m_Time = 0.0f;
 };