fn cell_key_multi(
row_cats: &[CategoryId],
row_tuple: &[(ItemId, String)],
col_cats: &[CategoryId],
col_tuple: &[(ItemId, String)],
pinned: &[(CategoryId, ItemId)],
) -> CoordKeyExpand description
The sorted CoordKey for a cell when categories are STACKED on each axis:
row_cats[i] binds to row_tuple[i], col_cats[j] to col_tuple[j], plus
the pinned page dims. Tuples come from axis_tuples so they align with the
category list. The general form of cell_key.