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.

Start by locating the CI wheel-building matrix and the configuration for spaCy's C extensions. Audit the extensions for free-threaded Python assumptions, then add cp314t coverage and verify that the resulting wheels are built and included in future PyPI releases.

Written by the indexing model from the issue text.