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.cpp

@@ -1,5 +1,6 @@
 #include "DisconnectShaderAttrCommand.h"
 #include "../../utils/Logger.h"
+#include <pxr/usd/usd/editContext.h>
 #include <pxr/usd/usdShade/shader.h>
 #include <pxr/usd/usdShade/connectableAPI.h>
 #include <pxr/base/tf/token.h>
@@ -8,10 +9,12 @@ namespace UsdLayerManager {
 
 DisconnectShaderAttrCommand::DisconnectShaderAttrCommand(pxr::UsdStageRefPtr stage,
                                                          const pxr::SdfPath&  destNode,
-                                                         const std::string&   destInput)
+                                                         const std::string&   destInput,
+                                                         pxr::SdfLayerHandle  editLayer)
     : m_stage(stage)
     , m_destNode(destNode)
     , m_destInput(destInput)
+    , m_editLayer(editLayer)
     , m_description("Disconnect " + destNode.GetName() + "." + destInput)
 {
     if (!stage) return;
@@ -38,7 +41,13 @@ void DisconnectShaderAttrCommand::Execute() {
         if (!destShader) return;
         pxr::UsdShadeInput input = destShader.GetInput(pxr::TfToken(m_destInput));
         if (!input) return;
-        pxr::UsdShadeConnectableAPI::DisconnectSource(input);
+
+        if (m_editLayer) {
+            pxr::UsdEditContext ec(m_stage, m_editLayer);
+            pxr::UsdShadeConnectableAPI::DisconnectSource(input);
+        } else {
+            pxr::UsdShadeConnectableAPI::DisconnectSource(input);
+        }
     } catch (const std::exception& e) {
         LOG_ERROR(std::string("DisconnectShaderAttrCommand::Execute error: ") + e.what());
     }
@@ -53,8 +62,14 @@ void DisconnectShaderAttrCommand::Undo() {
 
         pxr::UsdShadeInput input = destShader.GetInput(pxr::TfToken(m_destInput));
         pxr::UsdShadeOutput output = sourceShader.GetOutput(pxr::TfToken(m_sourceOutput));
-        if (input && output)
+        if (!input || !output) return;
+
+        if (m_editLayer) {
+            pxr::UsdEditContext ec(m_stage, m_editLayer);
             pxr::UsdShadeConnectableAPI::ConnectToSource(input, output);
+        } else {
+            pxr::UsdShadeConnectableAPI::ConnectToSource(input, output);
+        }
     } catch (const std::exception& e) {
         LOG_ERROR(std::string("DisconnectShaderAttrCommand::Undo error: ") + e.what());
     }