Fix prim expiry crashes and add context menu hierarchy ops

Crash fixes (expired UsdPrim after namespace edits):
- RenderPrimNode: early-out after rename commit (renamed flag + column cleanup)
- RenderPrimNode: early-out after context menu op expires prim (prim.IsValid() guard)
- RenderContextMenu: EndPopup+return when Unparent/Group expires prim mid-popup
- Child iteration: snapshot GetChildren() into vector before recursing to guard
  against live iterator invalidation from stage mutations deeper in the tree
- Rename: firstFrame guard prevents spurious IsItemDeactivated cancel on
  SetKeyboardFocusHere activation frame (fixes second-rename not working)
- RenamePrimCommand: check RenamePrim() return value, log if rejected

New features:
- Context menu Add Child submenu: create typed child prim under selection
- Context menu Unparent to Root: move prim to stage root (undo-able)

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

src/core/commands/RenamePrimCommand.cpp

@@ -27,7 +27,10 @@ void RenamePrimCommand::Execute() {
         return;
     }
     UsdNamespaceEditor editor(m_stage);
-    editor.RenamePrim(prim, TfToken(m_newName));
+    if (!editor.RenamePrim(prim, TfToken(m_newName))) {
+        LOG_ERROR("RenamePrimCommand::Execute: RenamePrim() rejected for " + m_oldPath.GetString());
+        return;
+    }
     std::string whyNot;
     if (!editor.CanApplyEdits(&whyNot)) {
         LOG_ERROR("RenamePrimCommand::Execute: cannot rename " + m_oldPath.GetString() + ": " + whyNot);
@@ -45,7 +48,10 @@ void RenamePrimCommand::Undo() {
         return;
     }
     UsdNamespaceEditor editor(m_stage);
-    editor.RenamePrim(prim, TfToken(m_oldName));
+    if (!editor.RenamePrim(prim, TfToken(m_oldName))) {
+        LOG_ERROR("RenamePrimCommand::Undo: RenamePrim() rejected for " + m_newPath.GetString());
+        return;
+    }
     std::string whyNot;
     if (!editor.CanApplyEdits(&whyNot)) {
         LOG_ERROR("RenamePrimCommand::Undo: cannot rename back " + m_newPath.GetString() + ": " + whyNot);