audio-fade-separate-gain-line

Implement a programmatic audio fade — a sleep timer, an alarm ramp, a duck — on a gain line of its own instead of the user's volume, and restore that gain from the player's own completion path. Use when a fade drags the volume slider down in the UI, when the app comes back permanently silent with a full slider, when a fade still ends in an audible click, or when audio briefly swells back after a fade completes.