Egui::Inspector::TabPart

The StyledPart behind the panel's own header tab cells (see widgets/styled_part.cr): the cells are paint-in-place — fixed-size rects + painter calls, not Ui#add — so without a part the inspector records no meta for them and a right-click pick opened an EMPTY menu. The part gives the cells a kind, the inspector.tab class and its keys, read through the same cascade (class rules + per-element overrides) as real widgets.