refactor: ECS 迁移到 entt，移除自研 Registry

- Entity = entt::entity（内部自带版本号），Registry = entt::registry
- Scene 对外接口不变（AddComponent/GetComponent/Each/延迟销毁/UUID），
  Renderer 与 App 层零改动完成迁移
- Each 转发 entt view（自动最小池遍历），延迟销毁队列保留
- 删除自研 Registry.h/Entity.h

Engine/src/ECS/Scene.cpp

@@ -9,32 +9,33 @@ namespace Engine::ECS {
 
     Entity Scene::CreateEntity(std::string name)
     {
-        Entity e = m_Registry.Create();
+        Entity e = m_Registry.create();
         // IdComponent 自动挂载：每个实体出生即有稳定 UUID
-        m_Registry.Add<IdComponent>(e, IdComponent{UUID::New()});
+        m_Registry.emplace<IdComponent>(e, IdComponent{UUID::New()});
         if (!name.empty()) {
-            m_Registry.Add<NameComponent>(e, NameComponent{std::move(name)});
+            m_Registry.emplace<NameComponent>(e, NameComponent{std::move(name)});
         }
         return e;
     }
 
     Entity Scene::FindEntityByUUID(UUID uuid)
     {
-        if (!uuid.Valid()) return {};
+        if (!uuid.Valid()) return entt::null;
 
-        Entity found{};
-        m_Registry.Each<IdComponent>([&](Entity e, IdComponent& id) {
-            if (!found && id.uuid == uuid) found = e;
+        Entity found = entt::null;
+        m_Registry.view<IdComponent>().each([&](Entity e, IdComponent& id) {
+            if (found == entt::null && id.uuid == uuid) found = e;
         });
         return found;
     }
 
     void Scene::DestroyEntity(Entity e)
     {
-        if (!m_Registry.IsAlive(e)) {
-            ENGINE_FATAL("Scene::DestroyEntity: stale entity ({},{})",
-                         e.index, e.generation);
+        if (!m_Registry.valid(e)) {
+            ENGINE_FATAL("Scene::DestroyEntity: stale entity ({})",
+                         static_cast<u32>(e));
         }
+        // 延迟到帧尾，保证遍历期间池结构不变
         m_PendingDestroy.push_back(e);
     }
 
@@ -43,8 +44,8 @@ namespace Engine::ECS {
         if (m_PendingDestroy.empty()) return;
 
         for (Entity e : m_PendingDestroy) {
-            if (m_Registry.IsAlive(e)) {
-                m_Registry.Destroy(e);
+            if (m_Registry.valid(e)) {
+                m_Registry.destroy(e);   // entt 自动回收槽位并递增版本号
             }
         }
         m_PendingDestroy.clear();
@@ -53,7 +54,7 @@ namespace Engine::ECS {
     CameraComponent* Scene::GetPrimaryCamera()
     {
         CameraComponent* found = nullptr;
-        m_Registry.Each<CameraComponent>([&](Entity, CameraComponent& cc) {
+        m_Registry.view<CameraComponent>().each([&](Entity, CameraComponent& cc) {
             if (!found && cc.primary) found = &cc;
         });
         return found;