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/core/commands/LayerCommands.cpp

@@ -46,10 +46,10 @@ LayerRemoveCommand::LayerRemoveCommand(LayerManager* mgr, int index)
     , m_description("Remove Layer")
 {
     if (!mgr) return;
-    auto layers = mgr->GetLayerStack();
-    if (index >= 0 && index < static_cast<int>(layers.size())) {
-        m_savedPath   = layers[index].identifier;
-        m_description = "Remove Layer " + layers[index].displayName;
+    auto sublayers = mgr->GetSublayers();
+    if (index >= 0 && index < static_cast<int>(sublayers.size())) {
+        m_savedPath   = sublayers[index].identifier;
+        m_description = "Remove Layer " + sublayers[index].displayName;
     }
 }
 
@@ -81,22 +81,14 @@ void LayerReorderCommand::Undo()    { ApplyOrder(m_before); }
 
 void LayerReorderCommand::ApplyOrder(const std::vector<std::string>& order) {
     if (!m_mgr) return;
-    // Remove all sublayers and re-insert in the desired order.
-    // We only control sublayers — root and session are fixed.
-    // First gather current sublayer indices (non-root, non-session).
-    auto layers = m_mgr->GetLayerStack();
 
-    // Count sublayers and remove them from highest index down.
-    std::vector<int> sublayerIndices;
-    for (int i = 0; i < static_cast<int>(layers.size()); ++i) {
-        if (!layers[i].isRootLayer && !layers[i].isSessionLayer)
-            sublayerIndices.push_back(i);
-    }
-    // Remove from back to front to preserve indices.
-    for (int i = static_cast<int>(sublayerIndices.size()) - 1; i >= 0; --i)
-        m_mgr->RemoveSublayer(sublayerIndices[i]);
+    // RemoveSublayer() operates on GetSubLayerPaths() — a sublayer-local list
+    // with indices 0..N-1.  GetLayerStack() prepends session and root layers, so
+    // its indices are always wrong here.  Use GetSublayers().size() for the count.
+    int count = static_cast<int>(m_mgr->GetSublayers().size());
+    for (int i = count - 1; i >= 0; --i)
+        m_mgr->RemoveSublayer(i);
 
-    // Re-add in desired order.
     for (const auto& path : order)
         m_mgr->InsertSublayerPath(path, -1);
 }