/**
 * The `split_by` corollary to [`GroupRollupMode`]. `Flat` (the default,
 * matching this crate's historical behavior) emits only full-depth split
 * combinations as columns; `Rollup` additionally emits grand-total and
 * subtotal column groups in "totals before" order. There is no `Total`
 * variant - an empty `split_by` already expresses a single grand-total
 * column group.
 */
export type SplitRollupMode = "flat" | "rollup";