RefCapture method
Copies the native struct o into a uniquely-keyed tracked slot and
returns its Dart-side X wrapper via ptrToDFunc.
Unique key of the form '<id>_<key>' is generated from the allocator's
ID counter. The returned X holds a live reference into temp-managed
memory.
Implementation
X RefCapture(String key, T o) => ptrToDFunc(setRefFunc(At('${_temp.nextId()}_$key'), o));