Dual-player crossfade

Two items must be audible at once, so a crossfade cannot be built on one player instance. The shape that works on both Android (Media3/ExoPlayer) and desktop engines:

• One player instance per media item. The audible one is currentPlayer; the one fading in is secondaryPlayer. Upcoming items may already exist as precached instances.
• One completion move for every early exit — pause, seek, skip: release the outgoing instance, promote the incoming one, restore its normal levels, clear the fading flag. Call it commit the incoming track as current. The natural finish usually keeps a near-twin of its own; hold the two in lockstep — every cleanup step added to one must appear in the other, or extract their shared tail.
• Equal-power (cosine/sine) gain curves, driven by one cancellable job.

val fadeAngle = (progress * PI / 2).toFloat()
currentPlayer?.volume = targetVolume * cos(fadeAngle)   // outgoing
nextPlayer.volume     = targetVolume * sin(fadeAngle)   // incoming