feat: support free-threaded CPython 3.14

[PyYoshi]

Summary

• declare the Cython extension free-threading compatible so importing it does not re-enable the GIL
• build and test cp314t wheels on Linux, macOS, and Windows
• define Py_GIL_DISABLED explicitly for Windows free-threaded builds
• verify the runtime GIL state in both the test suite and installed-wheel smoke test
• add a reproducible cChardet-only pyperf benchmark and publish before/after results

The existing Cython nogil regions intentionally remain. Under free-threaded CPython they still detach Python thread state around native-only work, and under regular CPython they are what allows independent detector calls to execute concurrently.

Performance

CPython 3.14.2 free-threaded, preloaded 160-file / 99,819-byte uchardet corpus, rigorous pyperf runs:

Workload	Before (module enables GIL)	After (GIL stays disabled)	Change
Serial corpus	30.6 ms	30.6 ms	no significant change
4-thread corpus	9.55 ms	9.25 ms	1.03x faster

Regular CPython 3.14.2 remained effectively unchanged: 30.2→30.3 ms serial and 9.39→9.38 ms with four threads.

Validation

• free-threaded CPython 3.14t imports with -Werror and leaves sys._is_gil_enabled() false
• 29 tests pass on CPython 3.11, 3.12, 3.13, 3.14, and free-threaded 3.14t
• concurrent exact-result stress test covers both detect() and detect_all()
• Ruff and strict mypy pass
• a real cp314-cp314t-linux_x86_64 wheel builds, installs into a clean 3.14t uv environment, and passes the wheel smoke test
• cibuildwheel 4.2.0 selects cp314t identifiers for Linux x86_64/aarch64, macOS x86_64/arm64, and Windows

References

• Cython free-threading guide: https://cython.readthedocs.io/en/latest/src/userguide/freethreading.html
• CPython extension guide: https://docs.python.org/3/howto/free-threading-extensions.html