GeezerVison

Fixes #161692.

In partial_vectorcall, pto_nkwds was counted before inspecting call keyword arguments. When overlapping keywords were merged into partial_keywords, custom __eq__ implementations on keyword names could execute arbitrary Python code and mutate p.keywords (e.g. adding thousands of entries or clearing the dictionary).

When keys were added, the subsequent PyDict_Next loop overfilled the fixed-size stack buffer / tuple (tot_kwnames), corrupting stack frames and causing SIGABRT (*** stack smashing detected ***). When keys were removed, assert(i == pto_nkwds) failed or null pointers were accessed.

Snapshotting partial_keywords via PyDict_Copy before key lookup and merging ensures the dictionary traversed for vectorcall arguments cannot be mutated during keyword merge.

• Issue: functools.partial: changing p.keywords during the call's keyword merge overflows a stack buffer #161692