Degrade gracefully when decode falls behind realtime

Pushback from #49, promoted to its own issue: under competing decode load (only reproducible under artificial contention; idle decode is 2.46x realtime) the wall-anchored presentation clock keeps its position, the picture drops frames, and the SOUND falls behind and goes silent - audio is the first casualty. Design question: an audio-mastered or waitable clock degrades better under starvation, but re-anchoring was deliberately rejected in #49 absent evidence. If starvation ever occurs in real use (thermal throttle, weaker hardware), this is where to look. Also from #49: absolute lip-sync rests on cpal/ALSA reporting device latency honestly; a constant bias is invisible to the drift instruments.