Trim to File Boundaries and Trim to Fill Selection extended a clip within
its source without looking at the clips around it, leaving two unfaded
clips over the same samples. Both now go through one helper that caps the
extension at the end of the previous clip and the start of the next one on
the same playlist, the way the other commands that grow an existing clip
do (Paste Merge MIDI, step input). An existing crossfade overlap is kept.