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/Editor.cpp

@@ -108,9 +108,8 @@ void Editor::DoStop()
 {
     if (!m_Ctx.scene || !m_Ctx.assets || !m_Ctx.renderer) return;
 
-    std::string envPath;
     if (!Serialization::SceneSerializer::DeserializeFromString(
-            *m_Ctx.scene, *m_Ctx.assets, m_PlaySnapshot, envPath))
+            *m_Ctx.scene, *m_Ctx.assets, m_PlaySnapshot))
     {
         ENGINE_ERROR("PIE: snapshot restore failed (scene is now the "
                      "played-out state)");
@@ -445,16 +444,8 @@ void Editor::DoSaveScene()
     std::filesystem::create_directories(
         std::filesystem::path(path).parent_path(), ec);
 
-    // 环境路径：引擎序列化器只认字符串，查表由编辑器侧负责
-    std::string envPath;
-    if (m_Ctx.getEnvIndex)
-    {
-        const int idx = m_Ctx.getEnvIndex();
-        if (idx >= 0 && idx < kEnvCount) envPath = kEnvPaths[idx];
-    }
-
     if (Serialization::SceneSerializer::Serialize(*m_Ctx.scene, *m_Ctx.assets,
-                                                  path, envPath))
+                                                  path))
     {
         ENGINE_INFO("Editor: scene saved to '{}'", path);
     }
@@ -472,10 +463,8 @@ void Editor::DoLoadScene()
         return;
     }
 
-    std::string envPath;
     if (!Serialization::SceneSerializer::Deserialize(*m_Ctx.scene,
-                                                     *m_Ctx.assets, path,
-                                                     envPath))
+                                                     *m_Ctx.assets, path))
     {
         ENGINE_ERROR("Editor: failed to load scene '{}'", path);
         return;
@@ -484,29 +473,4 @@ void Editor::DoLoadScene()
 
     // 旧实体已销毁：清选中（连带清描边，见 SelectEntity）
     SelectEntity(entt::null);
-
-    // 环境恢复：场景带环境且命中环境表才切换（镜像 StatsPanel 同步路径）
-    if (!envPath.empty() && m_Ctx.getEnvIndex && m_Ctx.setEnvIndex)
-    {
-        int found = -1;
-        for (int i = 0; i < kEnvCount; ++i)
-        {
-            if (envPath == kEnvPaths[i]) { found = i; break; }
-        }
-        if (found >= 0)
-        {
-            m_Ctx.setEnvIndex(found);
-            const auto env = m_Ctx.assets->LoadEnvironment(kEnvPaths[found]);
-            if (env.Valid())
-            {
-                m_Ctx.renderer->SetEnvironment(
-                    m_Ctx.assets->GetEnvironment(env));
-            }
-        }
-        else
-        {
-            ENGINE_WARN("Editor: scene env '{}' not in env table, "
-                        "keeping current", envPath);
-        }
-    }
 }