Variants emit natively from `python -m build` instead of being
post-processed onto an existing wheel. A new variant-build pixi feature
pulls scikit-build-core (henryiii fork backing PR #1284) plus
wheelnext/variantlib via pypi-deps; new opt-in CLI flags on
scripts/build_wheels.py thread the variant property + label through to
the existing `python -m build` invocations in
scripts/build_python_instance_base.py.
Activation:
  pixi run -e variant-macosx-py311 build-itk-wheels \
      --wheel-variant 'itk::threading::tbb' --wheel-variant-label tbbon
or via env vars (ITKPYTHONPACKAGE_WHEEL_VARIANT[_LABEL] /
ITKPYTHONPACKAGE_NULL_VARIANT) for CI parity with the existing
ITK_PACKAGE_VERSION / MANYLINUX_VERSION style. All flags default to off,
so production envs (manylinux228-py311, macosx-py311, ...) that don't
install the fork keep emitting their existing non-variant wheels and
never see the new config-settings.
Implementation:
- pixi.toml: new [feature.variant-build] (scikit-build-core fork +
  variantlib via pypi-deps), new [feature.variant-python-dev-pkgs]
  (mirror of python-dev-pkgs minus the conda scikit-build-core pin —
  pixi feature merging is intersection so the upper bound would
  otherwise prevent the fork from resolving), and four parallel
  environments: variant-{macosx,linux,manylinux228,windows}-py311.
- scripts/build_wheels.py: three new CLI flags (--wheel-variant,
  repeatable; --wheel-variant-label; --null-variant) with env-var
  defaults. Cross-flag validation (mutually exclusive null + property,
  property requires label) before the values land in
  package_env_config.
- scripts/build_python_instance_base.py: a small _variant_config_settings
  helper on BuildPythonInstanceBase that returns the right
  --config-setting=variant-* list (or [] for the production-default
  case). Spliced into both `python -m build` invocations
  (build_external_module_wheel and build_itk_python_wheels) just before
  echo_check_call. Validates the label and property regex up front so
  the build backend never sees an inconsistent state.
- experiments/wheel-variants/pixi.toml: drop variant-repack from
  pypi-deps (its CLI is broken end-to-end at experiment time — see
  findings.md). Phase-2 mock/ scripts retained as a reference.
Refs: scikit-build/scikit-build-core#1284, https://peps.python.org/pep-0817/