Fix nominal color scaling with integer levels #3949

Closes #3375.

This keeps nominal color scales aligned with their level order when integer color values are used across multiple layers. Coordinate scales still use matplotlib unit conversion so shared categorical axes keep their existing behavior.

Tests:
- python -m pytest tests/_core/test_plot.py::TestScaling::test_nominal_integer_color_scale_multiple_layers tests/_core/test_plot.py::TestScaling::test_pair_categories_shared tests/_core/test_scales.py::TestNominal -q
- python -m pytest tests/_core/test_scales.py tests/_core/test_properties.py::TestColor tests/_marks/test_dot.py -q
- python -m pytest tests/_core/test_plot.py -q -k "not test_show"
- python -m pytest tests/_core -q -k "not test_show"