A mapping layer made of extension functions

Two hops, two files, and neither of them knows anything about input or output:

transport payload  ──►  domain model  ──►  persistence row
   (data module,          (shared module, public extensions)
    internal extensions)

// data module — internal, so a transport type can never reach a screen
internal fun SongItem.toTrack(): Track = Track(…)

// shared module — public, both directions declared separately
fun Track.toSongEntity(): SongEntity = SongEntity(…)
fun SongEntity.toTrack(): Track = Track(…)