gh-102960: Make frames weak-referenceable (#152838)

gh-102960: Make frames weak-referenceable (#152838)

Add an explicit f_weakreflist field to the frame object, following the
same pattern as generators and coroutines, including free-threading-safe
weakref clearing via FT_CLEAR_WEAKREFS() in frame_dealloc().

Py_TPFLAGS_MANAGED_WEAKREF is not used because static builtin types
must not carry it (see init_static_type()) and the pre-header would
cost two extra words per frame instead of one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>