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.

まず、Python 3.14.2 の free-threading ビルドで import 警告を再現し、blosc2.blosc2_ext がどのようにビルドおよびロードされるかを調べます。py-free-threading.github.io のガイダンスを読みます。free-threaded ビルドで GIL を再有効化せずに blosc2 をインポートできれば完了です。

索引モデルが issue の本文から書いたものです。