Run existing pytest suites through Testenix

[polishdataengineer]

Summary

• add testenix pytest [PYTEST_ARGS ...] for unchanged pytest suites
• preserve pytest fixtures, conftest.py, parametrization, markers, classes, plugins, configuration, output, signals, and exit statuses
• use a POSIX process overlay and pytest's in-process console entry point on Windows
• add the optional testenix[pytest] extra for supported pytest releases (>=8.3,<10)
• document the exact native-versus-compatibility boundary in README, the website, CLI reference, benchmarks, and LLM-ready docs
• verify the built wheel and its pytest extra in CI and the release workflow

Why

The native testenix run collector intentionally does not emulate pytest internals. Simple pytest-style functions may overlap, but fixtures, hooks, plugins, skip/xfail, classes, and parametrization require real pytest semantics. This explicit bridge avoids silent partial compatibility while giving existing projects a zero-rewrite adoption path.

Native Testenix performance claims remain scoped to testenix run; testenix pytest delegates execution and does not receive native scheduler or reporting speedups.

Verification

• pytest: 86 passed
• testenix pytest -q: 86 passed
• testenix pytest -q -n 2: 86 passed
• exact pytest exit statuses verified for usage error (4) and no tests (5)
• Ruff, formatting, mypy, and pyright pass
• strict Sphinx build and generated LLM docs check pass
• wheel/sdist pass Twine and wheel-content checks
• isolated wheel succeeds with pytest 8.3.0 and 9.1.1
• isolated base wheel without pytest returns code 2 with the testenix[pytest] install hint
• zizmor --pedantic: no findings