Add Maya-style Render Layer panel

Render layers built on the existing USD sublayer architecture: each layer is a
file-backed SdfLayer mounted as a sublayer and tagged via custom layer data,
with membership stored in a UsdCollectionAPI on a hidden /RenderLayerData prim.
Activating a layer applies its overrides and isolates it to its members.
"Default" is virtual -- it means all render-layer sublayers muted.

See docs/adr/0002-render-layer.md for the design and rationale.

- RenderLayerManager: create/delete/rename/activate, membership, mute-based
  isolation. Sdf-level APIs are used while a layer is muted (Usd-level APIs
  can't see muted layers) and Usd-level APIs when active.
- Commands: create/delete/rename/activate and add/remove members, all undoable.
- RenderLayerPanel: layer list with one-click activation and a membership
  editor for the selected prims.
- StageEditorPanel / SceneHierarchyPanel: badge render-layer-owned sublayers,
  disable their mute/remove controls (they're managed from the Render Layer
  panel so the two can't desync), and lock the edit target while a non-Default
  layer is active.
- ConnectShaderAttrs/DisconnectShaderAttr: take an explicit editLayer captured
  at construction, so undo targets the same layer as execute even if the
  ambient edit target changed in between (e.g. a render-layer switch).

Note: SceneHierarchyPanel.h also carries declarations for the sublayers
section added in the following commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/Application.h

@@ -4,6 +4,7 @@
 #include "IconManager.h"
 #include "StageEditorPanel.h"
 #include "SceneHierarchyPanel.h"
+#include "RenderLayerPanel.h"
 #include "ViewportPanel.h"
 #include "PropertyPanel.h"
 #include "TimelinePanel.h"
@@ -11,6 +12,7 @@
 #include "MaterialEditorPanel.h"
 #include "../core/UsdStageManager.h"
 #include "../core/LayerManager.h"
+#include "../core/RenderLayerManager.h"
 #include "../core/PropertyManager.h"
 #include "../core/MaterialManager.h"
 #include "../core/CommandHistory.h"
@@ -65,6 +67,12 @@ private:
     void AddReferenceToStage();
     void CreatePrimOnStage(const std::string& typeName);
 
+    /// Explicitly saves every render-layer SdfLayer, since at most one is
+    /// ever in the stage's composition (unmuted) at a time and
+    /// UsdStage::Save() only walks layers currently in composition — the
+    /// rest would otherwise be silently dropped on save.
+    void SaveDirtyRenderLayers();
+
     // Playblast
     void CapturePlayblastFrame();
 
@@ -78,11 +86,13 @@ private:
     std::unique_ptr<IconManager> m_iconManager;
     std::unique_ptr<UsdStageManager> m_stageManager;
     std::unique_ptr<LayerManager> m_layerManager;
+    std::unique_ptr<RenderLayerManager> m_renderLayerManager;
     std::unique_ptr<PropertyManager> m_propertyManager;
     std::unique_ptr<MaterialManager> m_materialManager;
     CommandHistory m_commandHistory;
     std::unique_ptr<StageEditorPanel> m_stageEditorPanel;
     std::unique_ptr<SceneHierarchyPanel> m_sceneHierarchyPanel;
+    std::unique_ptr<RenderLayerPanel> m_renderLayerPanel;
     std::unique_ptr<ViewportPanel> m_viewportPanel;
     std::unique_ptr<PropertyPanel> m_propertyPanel;
     std::unique_ptr<TimelinePanel> m_timelinePanel;
@@ -91,6 +101,7 @@ private:
     bool m_showDemoWindow;
     bool m_showStageInfo;
     bool m_showStageEditor;
+    bool m_showRenderLayerPanel;
     bool m_showSceneHierarchy;
     bool m_showViewport;
     bool m_showPropertyPanel;