Enable free threading support

[vchamarthi]

Adds free-threaded (GIL-disabled) CPython 3.14t support to mkl_umath by declaring both the Cython extension and the hand-written _ufuncs C 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_umath/src/_patch_numpy.pyx as freethreading_compatible for Cython, and declare Py_MOD_GIL_NOT_USED in the _ufuncs C extension (ufuncsmodule.c) so importing either module no longer re-enables the GIL.
• Pin Cython to >=3.1.0 across build/packaging (pyproject.toml, both conda recipes, AGENTS.md, README.md).
• 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.
• Add concurrency tests covering multi-threaded ufunc execution and patch/restore state, plus a regression guard asserting the GIL stays disabled on import.

[ndgrigorian]

LGTM

[ndgrigorian]

@antonwolfy
if no more comments, I will go ahead and merge this

[antonwolfy]

LGTM