CPython 3.14.4 appended datastack_cached_chunk to PyThreadState, moving
_PyThreadStateImpl.c_stack_refs from offset 912 to 920 within the 3.14
series. One cp314t wheel serves every 3.14.x, so a wheel built for 3.14.4
or later read 8 bytes past the field on 3.14.0 through 3.14.3 and landed
on gc.alloc_count. 3.5.3 only copied that word around; 3.5.4 walks it as
a list, so it segfaults on the first switch.

Locate the field at import instead of trusting offsetof(). Reading back a
data descriptor puts us inside _PyObject_GenericGetAttrWithDict, which
has the real layout and holds a _PyCStackRef to that descriptor while it
calls __get__; the word of the thread state pointing at that node is
c_stack_refs. The search is bounded by two of our own stack markers and
matched against the descriptor. If it comes up empty on an interpreter we
were not built for, import fails instead of the first switch.

Tested on 3.14.6t, 3.15.0b2t, 3.14.6 and 3.13.14, plus wheels built for
3.14.6 run on 3.14.3 and the reverse.

Fixes python-greenlet#527