[b-re-w]

Closes #159

Defect fixed: importing any compiled TypedPython module on a free-threaded build turned the GIL back on for the whole process, because the module declared no Py_mod_gil slot. Without the GIL, the runtime's shared state also raced.

Feature added:

• Generated modules declare {Py_mod_gil, Py_MOD_GIL_NOT_USED} (under Py_GIL_DISABLED).
• Every piece of shared mutable state is classified (table in runtime/API.md, "Free-threaded CPython"):
  • thread-local: tp_depth, the poll countdown, tp_snapshot_depth;
  • atomic: the globals epoch, class guard tags and counters, and every tp_version_tag read;
  • immutable after init: the module state;
  • critical sections: list arguments of array parameters, held from copy-in to write-back on every exit path; the module dict for the epoch plus lookup and for the deopt counter.
• Borrowed references that could race were replaced by strong ones (PyDict_GetItemRef, PyEval_GetFrameBuiltins).
• A function with more than two list parameters stays interpreted with a reason, because CPython offers only one- and two-object critical sections.

Unchanged on purpose: functions holding an array snapshot still defer polls (#141), pending the decision in #152 on the changed-after-poll case. A long compiled array loop therefore delays other threads' stop-the-world until it returns. That is a liveness delay; results are unaffected.

Tests added: test_free_threaded.py, 23 tests.

• What they check:
  • the module declares the slot;
  • the GIL stays off after import;
  • no GIL RuntimeWarning;
  • concurrency in child interpreters: private and shared lists, read-modify-write against a serialisation, concurrent FieldSet/FieldGet, class modification while reading, global rebinding with the watcher, deopt counting.
• Red before the fix, under PYTHON_GIL=0: torn reads, a mis-rotated shared list, stale class bindings and lost deopt counts.
• Built-in mutation tests: removing the array critical section, or the module-dict critical section, makes them fail.
• TSan catches the epoch race but cannot see the list lock, because CPython's list code is not instrumented.

Tests changed:

• A type refcount is skipped when CPython 3.15t reports a deferred value; instance refcounts are still exact.
• A three-list sample became two lists.
• The deferred-handler test allows a 5 ms margin.
• tp_shim.c declares its GIL slot.

CI: typedpython.yml uses 3.15t in all three jobs. The Linux ASan job and the macOS app-fixture job have not run on 3.15t yet; they will run on the develop push.

Verification (local, CPython 3.15.0rc1 free-threaded, sys._is_gil_enabled() False): the full TypedPython suite with RuntimeWarning as error: 989 passed, 3 skipped. The branch also contains develop and #207.