free-threaded c_stack_refs offset between different python versions using the same wheel

[ddorian]

From #515 (comment)

3.14.4 appended datastack_cached_chunk to PyThreadState, moving _PyThreadStateImpl.c_stack_refs from 912 to 920 inside a released series. A greenlet wheel built for one 3.14.x read the wrong word on another, and 3.5.4 started dereferencing it. Rows 1 and 2 are that bug; rows 3 and 4 are the working tree resolving the offset at runtime instead.

❯ uv run --no-project freethread_abi_skew.py
Cross-version check for the free-threaded c_stack_refs offset (issue #515).
building the working tree for 3.14.6...
building the working tree for 3.14.3...

greenlet 3.5.5 wheel, on cpython-3.14.3+freethreaded
    3.14.3 | greenlet 3.5.5 | c_stack_refs at n/a
    -> segv

greenlet 3.5.5 wheel, on cpython-3.14.6+freethreaded
    3.14.6 | greenlet 3.5.5 | c_stack_refs at n/a
    switch ok
    -> ok

working tree built for 3.14.6, on cpython-3.14.3+freethreaded
    3.14.3 | greenlet 3.5.6.dev0 | c_stack_refs at 912
    switch ok
    -> ok

working tree built for 3.14.3, on cpython-3.14.6+freethreaded
    3.14.6 | greenlet 3.5.6.dev0 | c_stack_refs at 920
    switch ok
    -> ok

freethread_abi_skew.py

[ddorian]

Fix is in #528 (comment)