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/core/LayerManager.cpp

@@ -157,6 +157,24 @@ bool LayerManager::SetEditTarget(const std::string& identifier) {
     return true;
 }
 
+bool LayerManager::CreateNewSublayer(const std::string& absolutePath) {
+    if (!m_stage) return false;
+    SdfLayerHandle rootLayer = m_stage->GetRootLayer();
+    if (!rootLayer) return false;
+
+    auto newLayer = SdfLayer::CreateNew(absolutePath);
+    if (!newLayer) {
+        LOG_ERROR("Failed to create layer file: " + absolutePath);
+        return false;
+    }
+    newLayer->Save();
+
+    rootLayer->InsertSubLayerPath(absolutePath, 0);
+    m_stage->SetEditTarget(newLayer);
+    Refresh();
+    return true;
+}
+
 bool LayerManager::IsLayerMuted(const std::string& layerIdentifier) const {
     if (m_stage) {
         return m_stage->IsLayerMuted(layerIdentifier);

src/core/LayerManager.h

@@ -44,6 +44,9 @@ public:
     // Edit target
     bool SetEditTarget(const std::string& identifier);
 
+    // Create a new USD file on disk, insert as sublayer, and activate as edit target.
+    bool CreateNewSublayer(const std::string& absolutePath);
+
     // Muting
     void MuteLayer(const std::string& layerIdentifier);
     void UnmuteLayer(const std::string& layerIdentifier);