Add Maya-style prim editing to SceneHierarchyPanel

- Inline rename: double-click prim label or F2 to edit name in-place;
  Enter commits, click-away or Esc cancels (undo-able)
- Drag-and-drop reparent: drag any prim onto another to make it a child;
  invisible root drop zone below tree to reparent to stage root (undo-able)
- Ctrl+G group: creates Xform group at common parent, reparents selection
  under it; filters descendant duplication; selects new group (undo-able)
- Delete key shortcut: opens existing remove-prim modal when panel focused
- Context menu: Rename (F2) and Group (Ctrl+G) items added
- Three new undo-able commands using UsdNamespaceEditor (USD 25.05):
  RenamePrimCommand, ReparentPrimCommand, GroupPrimsCommand

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/SceneHierarchyPanel.h

@@ -114,6 +114,14 @@ private:
     std::string                  m_arcModalVarSetName;
     char                         m_arcModalBuf[512]   = {};
     bool                         m_openArcModal       = false;
+
+    /// Inline rename state.
+    SdfPath                      m_renamingPath;          ///< non-empty while renaming
+    char                         m_renameBuf[256]     = {};
+    bool                         m_renameJustStarted  = false;
+
+    void         HandleKeyboardShortcuts();
+    SdfPath      FindUniqueChildPath(const SdfPath& parent, const std::string& baseName);
 };
 
 } // namespace UsdLayerManager