[FEATURE] Publish free-threaded (no-GIL) CPython 3.14 wheels (cp314t)

Follow-up to #1875 (Part 2). #1875 fixed the GIL cp314 wheel (the aarch64 slot was
accidentally shipping cp314t), added a reserved cpy314t env spec (cpython-3.14t), a
create-recipe PATH fix, and a build-time ABI guard (_check-venv-abi). This issue
tracks intentionally building and publishing free-threaded cp314t wheels as a
deliberate, supported variant.

Scope

Publish cp314-cp314t wheels alongside the GIL cp314 wheels on all platforms
(Linux x86_64 / aarch64 manylinux, macOS arm64, Windows amd64).

Prerequisite: free-threading safety (do NOT skip)

Building a cp314t wheel merely declares free-threading support; it does not make the
extension safe. Before publishing cp314t as supported:

• Assess the NVX CFFI extensions (_utf8validator, _xormasker) for free-threading
  (no-GIL) safety — shared/global state, any implicit GIL assumptions, re-entrancy.
• Add the free-threaded module declaration so the interpreter does not silently re-enable
  the GIL on import (CPython's Py_mod_gil / the CFFI equivalent). Confirm
  sys._is_gil_enabled() stays False after import autobahn.nvx….
• Decide and document the support level (experimental vs supported).

Tasks

• Add cpy314t to the build matrix (ENVS / the wheels workflows). The _get-spec
  mapping cpy314t -> cpython-3.14t is already in place from #1875, and _check-venv-abi
  already validates the ABI (expects free-threaded for *t envs).
• Build cp314t on all four platforms; publish alongside cp314.
• Add cpy314t-<platform>-<arch> targets to the check-release-fileset targets in
  release.yml (the shared action supports the 314t token via wamp-cicd #11).
• Update docs/releases.rst wheel matrix and note the free-threading support level.

Related

• #1875 (GIL fix + reserved cpy314t spec + _check-venv-abi guard + PATH fix).
• wamp-cicd #11 (exact ABI-tag matching, so cp314 and cp314t slots are distinct).
• zlmdb # (the mirror for zlmdb; LMDB/CFFI free-threading assessment).

Note: This work was drafted with AI assistance (Claude Code). Initiated and reviewed by me.

Checklist

• I have searched existing issues to avoid duplicates
• I have described the problem clearly
• I have provided use cases
• I have considered alternatives
• I have assessed impact and breaking changes

Start with the ENVS and wheels workflows, then inspect the NVX CFFI extensions _utf8validator and _xormasker for free-threading safety and module declarations. Update release.yml targets and docs/releases.rst after confirming sys._is_gil_enabled() remains False on import. Done means cp314t wheels are built and published for all four platforms, with the support level documented.