Stage Editor Panel — sublayer editing with ET control, mute, reorder, dirty state

Replace LayerPanel with StageEditorPanel for full sublayer composition workflow:
- Edit target (ET) checkbox on root/session and each sublayer; pen icon header
- Dirty indicator (*) per layer; muted layers remain visible in list
- Drag-drop and Move Up/Down reordering with undo (LayerReorderCommand)
- Add existing sublayer via file dialog; create new sublayer modal
- Fixed LayerReorderCommand::ApplyOrder using full-stack indices on sublayer-local
  list (silent remove failure → duplicate insert → USD _ValidateEdit error)
- Fixed muted sublayer visibility: walk GetSubLayerPaths() + FindOrOpenRelativeToLayer
  + m_layerRefs to keep muted layers alive and visible

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/Application.h

@@ -2,7 +2,7 @@
 
 #include "ImGuiContext.h"
 #include "IconManager.h"
-#include "LayerPanel.h"
+#include "StageEditorPanel.h"
 #include "SceneHierarchyPanel.h"
 #include "ViewportPanel.h"
 #include "PropertyPanel.h"
@@ -49,7 +49,7 @@ private:
     std::unique_ptr<LayerManager> m_layerManager;
     std::unique_ptr<PropertyManager> m_propertyManager;
     CommandHistory m_commandHistory;
-    std::unique_ptr<LayerPanel> m_layerPanel;
+    std::unique_ptr<StageEditorPanel> m_stageEditorPanel;
     std::unique_ptr<SceneHierarchyPanel> m_sceneHierarchyPanel;
     std::unique_ptr<ViewportPanel> m_viewportPanel;
     std::unique_ptr<PropertyPanel> m_propertyPanel;