✨ feat(toml): declare factor group defaults and per-run overrides

[gaborbernat]

#4045 asks for two things: a default for a labelled factor group, so {factor:label} does not need {factor:label:fallback} spelled out at every use site, and a way to run the matrix against a value it does not list, sketched there as tox -f "django_version:'==6.0'" or TOX_F_django_version.

The default half becomes a table form for a labelled group, carrying either values or a range alongside default: { django_version = { values = ["django42", "django50"], default = "django50" } }. The default has to be one of the group's own factors, which is checked at load, so it can never resolve to a name no environment produces. It applies when no factor of the group is active in the current environment name, and a {factor:label:fallback} at the use site still wins over it.

The injection half arrives as TOX_FACTOR_<label>, read for labels the configuration declares. env TOX_FACTOR_django_version=django61 tox run -e django-py314-django50 runs the environment the matrix generated while {factor:django_version} resolves to django61. The literal syntax from the issue, a version specifier passed through -f, does not work in tox's model, since labelled factor values are environment-name components and >=4.2,<4.3 cannot be one. Keeping the override out of the name sidesteps that, at the cost that the environment name no longer tells the whole story of a run.

Closes #4045