Declare free-threaded (no-GIL) support for the _cchardet C extension

[wbarnha]

Summary

On free-threaded CPython (3.13t / 3.14t), importing cchardet re-enables the GIL because the _cchardet extension does not declare that it can run without it. The wheels build and the test suite passes, but the free-threading benefit is lost until the module opts in.

Evidence

Seen in cibuildwheel's cp314t (free-threaded) test step — the wheels build and test green, but with:

<frozen importlib._bootstrap>:491: RuntimeWarning: The global interpreter lock (GIL) has been
enabled to load module 'cchardet._cchardet', which has not declared that it can run safely
without the GIL. To override this behavior and keep the GIL disabled (at your own risk), run
with PYTHON_GIL=0 or -Xgil=0.

(cibuildwheel already produces cp314-cp314t manylinux + musllinux wheels once requires-python and cibuildwheel are current, so the only gap is the declaration.)

What's needed

1. Thread-safety review of the extension (src/cchardet/_cchardet.pyx + the vendored uchardet):
   • detect() creates a fresh uchardet_t per call and frees it — no shared state, so it looks inherently safe for concurrent calls.
   • UniversalDetector holds per-instance state; sharing one instance across threads is the caller's responsibility (same as on GIL builds) and should just be documented, not synchronized.
   • Confirm the vendored uchardet has no mutable process-global state on the detection path.
   • Module-level Python state is only the read-only _MAC_LABEL_ALIASES dict — safe.
2. Opt in via Cython. Cython 3.1+ supports the free-threading declaration; add the directive (e.g. # cython: freethreading_compatible = True) so the generated module sets Py_mod_gil = Py_MOD_GIL_NOT_USED. (We're already building with Cython 3.2.x.)
3. Verify the RuntimeWarning is gone and the suite passes under a free-threaded interpreter (e.g. run the cp313t/cp314t test path, or PYTHON_GIL=0 python -m pytest).

Acceptance criteria

• import cchardet on a free-threaded build no longer re-enables the GIL (no RuntimeWarning).
• Test suite passes on cp313t/cp314t with the GIL disabled.
• A short note in the README/docs on threading expectations for UniversalDetector (per-instance, not shared across threads).

Notes

• Not a blocker for the freedesktop-uchardet swap (Switch to upstream freedesktop uchardet + its candidate API (#46) #50); that PR already builds and tests the cp314t wheels, they just run with the GIL re-enabled.
• Low risk / self-contained, but the thread-safety review of the vendored C++ should be done before flipping the flag rather than assuming it's safe.