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/DisconnectShaderAttrCommand.h

@@ -3,6 +3,7 @@
 #include "../CommandHistory.h"
 #include <pxr/usd/usd/stage.h>
 #include <pxr/usd/sdf/path.h>
+#include <pxr/usd/sdf/layer.h>
 #include <string>
 
 namespace UsdLayerManager {
@@ -11,9 +12,13 @@ namespace UsdLayerManager {
 /// source at construction time so Undo can restore it.
 class DisconnectShaderAttrCommand : public ICommand {
 public:
+    /// editLayer: see ConnectShaderAttrsCommand's constructor doc — same
+    /// capture-at-construction-time pattern, null/empty falls back to the
+    /// ambient edit target at call time (today's behavior).
     DisconnectShaderAttrCommand(pxr::UsdStageRefPtr stage,
                                 const pxr::SdfPath&  destNode,
-                                const std::string&   destInput);
+                                const std::string&   destInput,
+                                pxr::SdfLayerHandle  editLayer = pxr::SdfLayerHandle());
 
     void Execute() override;
     void Undo()    override;
@@ -23,6 +28,7 @@ private:
     pxr::UsdStageRefPtr m_stage;
     pxr::SdfPath        m_destNode;
     std::string         m_destInput;
+    pxr::SdfLayerHandle m_editLayer;
     std::string         m_description;
 
     bool         m_hadConnection = false;