The owners of Boston’s commercial classical radio station, WCRB, are putting the station up for sale, but say they will require any buyer to maintain the station’s classical format on at least a digital side-channel accessible to listeners with high-definition receivers. That isn’t terribly reassuring to some listeners, since HD radios are almost entirely unknown in the U.S., and some in the industry question whether the format requirement actually carries any legal weight. Boston has two other stations that broadcast classical music, but WCRB is the only full-time classical station.