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/IconManager.h

@@ -50,6 +50,10 @@ enum class Icon {
     SkipEnd,      // go to last frame   (▶|)
     Loop,         // loop toggle        (↻)
     Bounce,       // bounce/ping-pong   (↔)
+    // General actions
+    Refresh,      // reload / refresh
+    FilePlus,     // create new file
+    Pen,          // edit / edit target
 };
 
 /// Loads SVG files from disk, rasterizes them with NanoSVG, uploads them as