str.__hash__ can’t be overridden, so for exact PyStr instances we
skip the attribute lookup and MRO walk and call the string hash directly.
This reduces overhead on hot paths (e.g. dict/set key hashing) while keeping
subclass semantics intact (downcast_ref_if_exact).

Reduce calls and lock acquisitions on hot paths by bypassing get_class_attr(__hash__) and directly resolving the hash implementation with mro_find_map(|cls| cls.slots.hash.load()).