Add support for the free-threaded build

blosc2 ships free-threaded wheels, but they re-enable the GIL on import:

```
goldbaum at Nathans-MBP in ~
○ python
Python 3.14.2 free-threading build (main, Dec 8 2025, 15:41:12) [Clang 17.0.0 (clang-1700.4.4.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
i>>> import blosc2
:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'blosc2.blosc2_ext', 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.
```

See https://py-free-threading.github.io for more information about the free-threaded build and how to support it.

Start by reproducing the import warning with the Python 3.14.2 free-threading build and inspect how blosc2.blosc2_ext is built and loaded. Read the guidance at py-free-threading.github.io; done means importing blosc2 without re-enabling the GIL on a free-threaded build.

Written by the indexing model from the issue text.