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/core/commands/ConnectShaderAttrsCommand.h

@@ -4,6 +4,7 @@
 #include <pxr/usd/usd/stage.h>
 #include <pxr/usd/sdf/path.h>
 #include <pxr/usd/sdf/valueTypeName.h>
+#include <pxr/usd/sdf/layer.h>
 #include <string>
 
 namespace UsdLayerManager {
@@ -14,13 +15,19 @@ namespace UsdLayerManager {
 /// Undo can restore it exactly rather than merely disconnecting.
 class ConnectShaderAttrsCommand : public ICommand {
 public:
+    /// editLayer: layer to author into (mirrors TransformCommand's pattern).
+    /// Captured at construction time so Undo() targets the same layer as
+    /// Execute() even if the stage's ambient edit target changes in between
+    /// (e.g. a render-layer switch) — pass null/empty to fall back to
+    /// whatever the ambient edit target is at call time (today's behavior).
     ConnectShaderAttrsCommand(pxr::UsdStageRefPtr stage,
                               const pxr::SdfPath&  destNode,
                               const std::string&   destInput,
                               const pxr::SdfValueTypeName& destType,
                               const pxr::SdfPath&  sourceNode,
                               const std::string&   sourceOutput,
-                              const pxr::SdfValueTypeName& sourceType);
+                              const pxr::SdfValueTypeName& sourceType,
+                              pxr::SdfLayerHandle  editLayer = pxr::SdfLayerHandle());
 
     void Execute() override;
     void Undo()    override;
@@ -34,6 +41,7 @@ private:
     pxr::SdfPath          m_sourceNode;
     std::string           m_sourceOutput;
     pxr::SdfValueTypeName m_sourceType;
+    pxr::SdfLayerHandle   m_editLayer;
     std::string           m_description;
 
     bool        m_hadPriorConnection = false;