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/main.cpp

@@ -27,7 +27,7 @@
 #include <GLFW/glfw3.h>
 
 #include "Editor.h"
-#include "EditorContext.h"   // kEnvPaths / kDefaultEnv（与 StatsPanel 共用）
+#include "EditorContext.h"
 
 using namespace Engine;
 
@@ -117,8 +117,6 @@ namespace
             ctx.assets = &GetAssets();
             ctx.app = this;
             ctx.editorCamera = &GetRenderer().GetEditorCamera();
-            ctx.getEnvIndex = [this] { return m_EnvIndex; };
-            ctx.setEnvIndex = [this](int index) { m_EnvIndex = index; };
             // 继承原沙盒默认：初始暂停（StatsPanel 的 Pause Simulation 开关恢复）
             Time::SetTimeScale(0.0f);
             m_Editor->Init(ctx);
@@ -135,7 +133,6 @@ namespace
 
     private:
         std::unique_ptr<Editor> m_Editor = std::make_unique<Editor>();
-        int m_EnvIndex = kDefaultEnv;
     };
 }