Module header 

Sticky header strip + per-column header cells.

HeaderCell lays out as Padding → HStack { TextWidget(label), Spacer, SortIndicator? } and handles click-to-sort plus drag-to-resize on the grip that straddles either of the two column dividers it touches (see the type’s docs). HeaderRow lays its cells horizontally using the same shared column_widths handle that body rows consume — so a resize commits in one place and reflows everywhere — and paints the column separators that make those grips findable.

Supports sort, resize, reuse across pinned panes, per-column filter popovers, and column-reorder drag.