Add cp314t (free-threaded Python 3.14) wheel support

Summary

Request to add pre-built wheels for free-threaded Python 3.14 (cp314t) to PyPI releases.

Context

• Discussion tracking community interest: #13995
• cp314 regular wheels are already built but missing from PyPI: #13949
• Other key ecosystem packages (numpy, pydantic-core, uvloop, greenlet) have already shipped cp314t wheels, making spaCy one of the last blockers for teams wanting to run on free-threaded Python 3.14

What's needed

1. Mark spaCy's C extensions as free-threaded safe (or audit for GIL assumptions)
2. Add cp314t to the CI wheel-building matrix
3. Upload cp314t wheels to PyPI alongside regular cp314 wheels in future releases

Why this matters

Teams using spaCy in ML/NLP pipelines cannot migrate to python:3.14t (free-threaded CPython) until spaCy ships cp314t wheels. Since the rest of the major deps have already committed to cp314t, spaCy is the blocking dependency.