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>
