Support free-threaded CPython 3.14

[Kludex]

Summary

• add CI coverage for regular CPython 3.14 and free-threaded 3.14t on Linux and macOS
• avoid private free-threaded CPython symbols by using public refcount and type-flag APIs on 3.14t
• load the extension under CPython's compatibility GIL and document that behavior
• package only the extension matching the wheel's active ABI

Root cause

The free-threaded refcount and type-check macros expanded to private CPython helpers, leaving unresolved symbols such as __Py_ThreadId and _Py_atomic_load_uint64_relaxed in the extension. The wildcard wheel artifact rule could also collect stale extensions built for other Python ABIs.

Validation

• CPython 3.14: 274 passed, 1 expected 3.14t-only skip; 100% coverage
• CPython 3.14t: 275 passed; 100% coverage
• Ruff check and format check
• mypy strict type check
• strict documentation build
• isolated build and import of cp314 and cp314t wheels; each wheel contains only its matching extension