ci: build free-threaded manylinux x86_64 wheel

[honglei]

This adds an explicit Linux x86_64 free-threaded wheel build for Python 3.14t.

The v0.3.6 release artifacts include the regular CPython 3.14 manylinux x86_64 wheel, but not the free-threaded one:

• present: fastcrc-0.3.6-cp314-cp314-manylinux_2_17_x86_64.manylinux2014_x86_64.whl
• missing: fastcrc-0.3.6-cp314-cp314t-*-x86_64.whl for glibc/manylinux

There is a cp314t x86_64 musllinux wheel, and other Linux architectures already have cp314t manylinux wheels. The missing artifact appears to be limited to the glibc x86_64 free-threaded wheel.

The new job keeps the existing manylinux2014 compatibility baseline and adds a dedicated Python 3.14t x86_64 build using:

• manylinux: 2014
• args: --release --out dist -i python3.14t

The release job now depends on this new artifact so tag releases can publish it together with the existing wheels.

Validation performed locally:

• inspected the v0.3.6 wheels-linux-x86_64 artifact from GitHub Actions; it contains cp314-cp314 but no cp314-cp314t wheel
• verified quay.io/pypa/manylinux2014_x86_64 currently contains both cp314-cp314 and cp314-cp314t under /opt/python
• built fastcrc 0.3.6 locally for /opt/python314t as fastcrc-0.3.6-cp314-cp314t-manylinux_2_34_x86_64.whl and imported it successfully in a free-threaded Python 3.14t runtime

This PR is intentionally kept as a draft until CI confirms the additional manylinux2014 job produces the expected cp314-cp314t x86_64 wheel.