Feat: Add free-threaded Python support

[vchamarthi]

Summary

Enable free-threaded Python support for mkl_fft.

The Meson migration in #303 is now merged, so this PR contains only the
changes required for free-threaded support.

Depends on IntelPython/mkl-service#213 for a compatible free-threaded
mkl-service package.

[Copilot]

Pull request overview

This PR enables free-threaded (PEP 703) Python support for mkl_fft by marking the Cython extension as free-threading compatible, aligning build tooling requirements, and expanding CI/conda packaging to cover both CPython 3.14 (GIL) and 3.14t (free-threaded) variants.

Changes:

• Mark mkl_fft/_pydfti.pyx as freethreading_compatible for Cython and pin Cython to >=3.1.0 across build/packaging.
• Update conda recipes to require cython >=3.1.0 and remove python-gil pins.
• Extend conda/pip CI matrices to build/test CPython 3.14 cp314 and cp314t variants and adjust cache/artifact naming accordingly.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pyproject.toml	Pins build-time Cython to >=3.1.0 for free-threading support features.
mkl_fft/_pydfti.pyx	Declares the extension as free-threading compatible via a Cython directive.
conda-recipe/meta.yaml	Aligns Intel-channel recipe with Cython >=3.1.0 and drops python-gil pins.
conda-recipe-cf/meta.yaml	Aligns conda-forge recipe with Cython >=3.1.0 and drops python-gil pins.
AGENTS.md	Updates documented build dependencies to reflect the Cython pin.
.github/workflows/conda-package.yml	Adds 3.14 cp314 and cp314t build/test coverage and distinguishes variants in cache/artifacts.
.github/workflows/conda-package-cf.yml	Mirrors conda-forge CI to build/test both 3.14 variants and adjust cache/artifacts.
.github/workflows/build-with-standard-clang.yml	Adds 3.14t to the Python matrix (currently incompatible with actions/setup-python).
.github/workflows/build-with-clang.yml	Adds 3.14t to the Python matrix (currently incompatible with actions/setup-python).
.github/workflows/build_pip.yml	Adds 3.14 cp314/cp314t variants via a python_spec matchspec approach.