Summary

Replace the general-purpose set_swap_bodies() with a specialized set_replace_body() that exploits the invariant that the source argument is always a uniquely-referenced temporary set about to be discarded.

Follow-up to the observation in #132290 (comment) and #142659.

Changes

set_swap_bodies() was designed for arbitrary two-way swaps between any two sets, but it is only called from set_intersection_update() and set_intersection_update_multi_impl(), where the second argument (tmp) is always a freshly created temporary with Py_REFCNT == 1.

The new set_replace_body() exploits this invariant:

• Skip atomic stores on src: the temporary is not visible to other threads, so plain assignments suffice (saves atomic fence overhead in the free-threaded build).
• Skip copy_small_table for src: use plain memcpy instead of per-entry atomic stores when writing back to src's smalltable (Py_GIL_DISABLED path).
• Remove frozenset hash swap: neither argument is ever a frozenset in these call sites (enforced via assert).
• Simplify shared-marking logic: src is never shared (enforced via assert), so only one direction of the shared-marking check is needed — propagate shared status from dst to src for proper deallocation of old entries.
• Remove unused Py_hash_t h variable.

All assumptions are guarded by assert() to document and enforce the contract.

• Issue: Minor optimization for set object, swap bodies #142659