Under free-threaded CPython (Py_GIL_DISABLED) the GIL provides no mutual
exclusion, so the plain pointer last_storage_ptr_ was read and written
concurrently without synchronization, a C++ data race. Make it a
std::atomic<T *>.
Also reorder get_stored() to check is_last_storage_valid() before loading
the cached pointer. The writer publishes the pointer before setting the
validity flag, so the flag must be observed first for correct
acquire/release ordering.
Assisted-by: ClaudeCode:claude-fable-5
085b66068a