a super-header spanning its child columns — renders a multi-row header where header sits above
its columns with a colSpan. Children can be leaf columns OR other groups (nested to any depth),
so a 3+ level header (e.g. Year → Half → Quarter) is just groups inside groups.
a super-header spanning its child columns — renders a multi-row header where
headersits above itscolumnswith a colSpan. Children can be leaf columns OR other groups (nested to any depth), so a 3+ level header (e.g. Year → Half → Quarter) is just groups inside groups.