GeezerVison

Summary

Refs #161850.

Serialize forward and reverse deque iteration, and iterator reduction, using the deque's mutex alone. The iterator retains its deque throughout its lifetime, and its mutable iteration state is protected by that mutex. This removes the iterator mutex acquisition/release from the per-item path.

Changes

• Use the deque's critical section in dequeiter_next(), dequereviter_next() and dequeiter_reduce().
• Use relaxed atomic stores for the remaining-item count, paired with the unlocked __length_hint__ read.
• Add shared forward/reverse iterator and concurrent mutation tests in test_free_threading.test_collections.
• Add the NEWS entry for Free-threaded build: deque iteration and list(deque) take a critical section per item #161850.

Test Plan

• Source review and git diff --check pass for this main-based branch.
• The main-based CI run passed all 48 jobs on head d0ed2da495922d2887672d3fd0c2cdeb22df4278, including ordinary/free-threaded builds and TSan jobs.
• Performance measurements below were run on a CPython 3.14.6 backport, not this main-based (3.16 development) port. This PR stays draft for core review.

On the tested 950, the 3.14.6 backport's 1000-element deque iteration benchmark decreased from about 52.3 to 30.6 microseconds. This is supporting evidence for the approach, not a performance claim for this branch.

AI tools assisted preparation of the change and submission text.