Function arrangement_sizes_expired_retractions 

fn arrangement_sizes_expired_retractions(
    rows: impl IntoIterator<Item = (Row, i64)>,
    cutoff_ts: u128,
    item_id: CatalogItemId,
) -> Vec<BuiltinTableUpdate>

Returns retraction updates for rows in a consolidated mz_object_arrangement_size_history snapshot whose collection_timestamp (column 3) is strictly before cutoff_ts.

Every input row should have diff == 1: the caller must consolidate first, this function is the only source of retractions, and a single writer never repeats a collection_timestamp. Any other diff is a soft panic, and the expired row is retracted at its full multiplicity. A hard panic here would fail every boot, since this runs at startup.