Fix categorical hue splits with explicit hue order (#3922) #3971

Closes #3922.

Fixes an issue where categorical hue data could be split incorrectly when the pandas categorical dtype order differed from hue_order. In that case, plots such as violinplot could show the right hue labels/colors but draw them with the wrong subset of data.

The fix updates the shared iter_data splitter to match grouping keys by value directly instead of relying on GroupBy.get_group.

Tests added for the low-level splitter and violinplot.