Scene Hierarchy ET switcher + Create Sublayer via Save dialog

- Add edit-target layer combo at top of Scene Hierarchy panel (always
  visible when stage loaded — session, root, sublayers); routes through
  LayerManager::SetEditTarget so StageEditor ET checkboxes stay in sync
- Replace custom Create Sublayer modal with native SaveFile dialog;
  LayerManager::CreateNewSublayer creates the file on disk, inserts it
  at the top of the sublayer stack, and activates it as edit target

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

src/ui/SceneHierarchyPanel.h

@@ -2,6 +2,7 @@
 
 #include "../core/PropertyManager.h"
 #include "../core/CommandHistory.h"
+#include "../core/LayerManager.h"
 #include "IconManager.h"
 #include <pxr/usd/usd/stage.h>
 #include <pxr/usd/sdf/path.h>
@@ -24,6 +25,7 @@ public:
     
     void SetPropertyManager(PropertyManager* manager);
     void SetCommandHistory(CommandHistory* history) { m_commandHistory = history; }
+    void SetLayerManager(LayerManager* lm) { m_layerManager = lm; }
     void SetStage(UsdStageRefPtr stage);
     void SetIconManager(IconManager* iconManager) { m_iconManager = iconManager; }
     void Render();
@@ -74,6 +76,7 @@ private:
     
     PropertyManager*             m_propertyManager;
     CommandHistory*              m_commandHistory = nullptr;
+    LayerManager*                m_layerManager   = nullptr;
     IconManager*                 m_iconManager = nullptr;
     UsdStageRefPtr               m_stage;