Choppy persisted after NACK rollback. Dial back to 320k (transparent
threshold, much less stress on uploads alongside screen+camera). Drop
a=maxptime:120 — receivers + 700ms playout buffer don't handle larger
frames well. Fix the SDP regex that silently no-op'd on Firefox
publishers (Firefox's Opus fmtp lacks minptime=10 — the matcher
required it). Firefox publishers will now actually carry the music
codec params.