refactor(editor): isolate scene backend boundary

editor/app/Services/Scene/EditorSceneRuntime.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "Scene/EditorSceneBridge.h"
+#include "Scene/EditorSceneBackend.h"
 #include "Scene/SceneViewportCameraController.h"
 #include "Scene/SceneToolState.h"
 #include "State/EditorSelectionService.h"
@@ -55,6 +55,7 @@ public:
     EditorSceneRuntime& operator=(EditorSceneRuntime&&) = delete;
 
     void Reset();
+    void SetBackend(std::unique_ptr<EditorSceneBackend> backend);
     bool Initialize(const std::filesystem::path& projectRoot);
     void BindSelectionService(EditorSelectionService* selectionService);
 
@@ -175,6 +176,7 @@ private:
     void IncrementInspectorRevision();
 
     std::filesystem::path m_projectRoot = {};
+    std::unique_ptr<EditorSceneBackend> m_backend = {};
     EditorStartupSceneResult m_startupSceneResult = {};
     EditorSelectionService m_ownedSelectionService = {};
     EditorSelectionService* m_selectionService = &m_ownedSelectionService;