Holdout rows per label (label as text → row count) for a classification carve; an empty object for regression
or when the label column is absent. Training stores it in the model Lineage as lockedHoldoutClassCounts.
LockedHoldout.rows.length.
The rows scored exactly once, never trained on.
The rows the model may train and tune on.
Training.rows.length.
The result of carving a matrix into a training portion and a locked holdout; both keep the input row order.