Folds table_rows into heights for each
Lookup-kind spec, taking the running max.
Used to derive the per-bus N_max absorbed
into the transcript before r_bus is drawn.
Every multi-endpoint Permutationbus_id
must have at least one endpoint owning a real
RowIndexLeBytes/RowIndexByte clock;
otherwise label-only stitching admits
char-2 parity collapse.