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/RenderLayerMembershipCommand.h

@@ -0,0 +1,44 @@
+#pragma once
+
+#include "../CommandHistory.h"
+#include "../RenderLayerManager.h"
+#include <pxr/usd/sdf/path.h>
+#include <string>
+#include <vector>
+
+namespace UsdLayerManager {
+
+/// Adds one or more prim paths to a render layer's membership (Sdf-level, so
+/// this works whether or not layerId is currently active). Re-runs viewport
+/// isolation if layerId happens to be the active layer.
+class RenderLayerAddMembersCommand : public ICommand {
+public:
+    RenderLayerAddMembersCommand(RenderLayerManager* mgr, std::string layerId,
+                                 std::vector<pxr::SdfPath> paths);
+    void Execute() override;
+    void Undo()    override;
+    std::string GetDescription() const override { return m_description; }
+
+private:
+    RenderLayerManager*       m_mgr;
+    std::string                m_layerId;
+    std::vector<pxr::SdfPath>  m_paths;
+    std::string                m_description;
+};
+
+class RenderLayerRemoveMembersCommand : public ICommand {
+public:
+    RenderLayerRemoveMembersCommand(RenderLayerManager* mgr, std::string layerId,
+                                    std::vector<pxr::SdfPath> paths);
+    void Execute() override;
+    void Undo()    override;
+    std::string GetDescription() const override { return m_description; }
+
+private:
+    RenderLayerManager*       m_mgr;
+    std::string                m_layerId;
+    std::vector<pxr::SdfPath>  m_paths;
+    std::string                m_description;
+};
+
+} // namespace UsdLayerManager