The Zig-generated dist/core.c is rejected under C23 (GCC 15's default),
so pin C11 in the native extension build.

Refs COD-3244

Co-authored-by: Claude <[email protected]>

Print a warning in the pytest header when the interpreter is outside the version
range pytest-codspeed claims to support, so customers upgrading Python are told
their measurements run on an untested interpreter instead of getting silent,
unvalidated results.

On GitHub Actions the warning is emitted as a `::warning` workflow command so it
becomes an annotation shown outside the job log; on GitLab CI, which has no
annotation mechanism, the line is coloured instead. Mirrors the `warnCi` helper
of the codspeed-node counterpart, including the percent-encoding of
workflow-command syntax.

The bounds mirror `requires-python` and the `Programming Language :: Python`
classifiers in pyproject.toml, and the CI matrix.

Refs COD-3400

Python 3.15.0 is now available as a stable release in actions/setup-python, so replace the 3.15.0-beta.1 matrix entries with 3.15 and 3.15t. The free-threaded build is selected with the t suffix, like 3.14t, so the architecture matrix field is no longer used and is removed from the setup step.

pytest 9.2 replaced the private `FixtureManager._arg2fixturedefs` with
`_arg2node2fixturedefs` (pytest-dev/pytest#14984). The
`pytest_plugin_registered` hook wrote into `_arg2fixturedefs` to swap
pytest-benchmark's `benchmark` fixture for ours, so every `--codspeed`
run with pytest-benchmark installed crashed with an AttributeError.

On pytest 9.1+, register the override through the public
`pytest.register_fixture` API at session start instead. Older pytest
keeps the previous `_arg2fixturedefs` swap.

pytest-benchmark itself still reads the removed `Node._nodeid` on
pytest 9.2, so the two tests that run it without `--codspeed` are
marked as non-strict xfail on that combination.

CI now also runs pytest ~=9.0.0 and ~=8.0.0 on the latest Python to
cover the legacy override paths, and installs valgrind for the
pytest-benchmark-4/5 configs, which the old `== 'pytest-benchmark'`
check never matched.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

tests/examples holds files that other tests copy through pytester. Collected
directly, they run pytest-benchmark's own fixture, which reads `node._nodeid`,
removed in pytest 9.2 and still read by pytest-benchmark 5.3.0. The wheel tests
install the latest pytest, so the v5.0.4 release build failed. The local pytest
config already ignores tests/examples.

Refs COD-4044

Co-authored-by: Claude Opus 5.5 <[email protected]>