Add Python 3.14 free-threaded (cp314t) compatibility

[clemlesne]

Summary

With the release of Python 3.14, the new free-threaded (no-GIL) mode is becoming more widely used. It would be great to confirm bm25s compatibility with python3.14t (the free-threaded ABI).

Current State

• bm25s is primarily pure Python with scipy/numpy dependencies
• numpy 2.3+ and scipy 1.16+ now have cp314t wheels
• Unknown if bm25s works correctly under free-threaded Python

What's Needed

1. Add Python 3.14t to CI test matrix
2. Verify thread safety under free-threaded Python (if using any native extensions)
3. Update PyPI classifiers to indicate free-threaded support once verified

Resources

• Python Free-Threading Guide
• Free-Threaded Wheels Tracker
• PEP 703 – Making the GIL Optional

[xhluca]

Good idea. happy to review a PR if you are interested in adding tests/fix for this.