Environment settings become scene entities (UE Skybox/SkyLight semantics)

Skybox/SkyLight are now Actor-carried components instead of resident
global settings, matching UE where SkyAtmosphere/SkyLight/fog are
Actors added from Place Actors and configured in Details.

- New SkyboxComponent (active + envPath asset slot) and SkyLightComponent
  (active + ambientColor); first-active entity wins per frame
- SceneRenderer collects env from components into FrameSceneData
  (DrawSkybox/envPath/ambient); Renderer performs delta sync - env
  reloads only when the resolved path changes, neutral fallback when
  empty or unloadable
- App-side env plumbing removed: kEnvPaths table and getEnvIndex/
  setEnvIndex callbacks deleted; StatsPanel keeps ClearColor + debug
  view, env params move to Details
- Serializer: scene-level Settings stores ClearColor only; env
  components serialize per entity; legacy v1 scenes (Environment header
  + Settings.AmbientColor/DrawSkybox) auto-migrate into an Environment
  entity on load for visual parity
- Inspector: Skybox/Sky Light sections with .hdr asset slot dropdown
  and Add Component entries; Outliner "+ Create" popup offers
  Empty/Light/Camera/Environment

App/editor/HierarchyPanel.cpp

@@ -148,11 +148,39 @@ void HierarchyPanel::Draw(const EditorContext& ctx, entt::entity& selected)
 {
     ImGui::Begin("Outliner");
 
-    // 工具栏：创建实体（在遍历之前，不修改遍历中的注册表结构）
-    if (ImGui::Button("+ Create Entity"))
+    // 工具栏：创建实体（在遍历之前，不修改遍历中的注册表结构）。
+    // Place Actors 简化语义：常用类型一键带组件，其余 Add Component 补
+    if (ImGui::Button("+ Create"))
     {
-        const auto e = ctx.scene->CreateEntity("Entity");
-        selected = e;
+        ImGui::OpenPopup("create_entity");
+    }
+    if (ImGui::BeginPopup("create_entity"))
+    {
+        if (ImGui::MenuItem("Empty"))
+        {
+            selected = ctx.scene->CreateEntity("Entity");
+        }
+        if (ImGui::MenuItem("Light"))
+        {
+            auto e = ctx.scene->CreateEntity("Light");
+            e.AddComponent<Engine::ECS::LightComponent>();
+            selected = e;
+        }
+        if (ImGui::MenuItem("Camera"))
+        {
+            auto e = ctx.scene->CreateEntity("Camera");
+            e.AddComponent<Engine::ECS::CameraComponent>();
+            selected = e;
+        }
+        if (ImGui::MenuItem("Environment"))
+        {
+            // UE 新关卡环境集简化：Skybox（背景）+ SkyLight（环境光）一体
+            auto e = ctx.scene->CreateEntity("Environment");
+            e.AddComponent<Engine::ECS::SkyboxComponent>();
+            e.AddComponent<Engine::ECS::SkyLightComponent>();
+            selected = e;
+        }
+        ImGui::EndPopup();
     }
     ImGui::Separator();