Fix transient crashes during audio-engine recovery

[sanylax0]

Root cause

A configuration change can stop AVAudioEngine during a crossfade without sending the interruption path through pauseForEnvironment(). Recovery rescheduled only the outgoing deck while leaving the transition active and the incoming deck's invalidated schedule reachable. A later pause/resume could call play() on that stale deck, triggering an uncatchable AVFAudio exception; media-services reset also left skip-transition flags orphaned.

Fix

• Discard an interrupted transition before rescheduling the outgoing deck.
• Clear transition state without touching orphaned AVAudio nodes after media-services reset or a failed engine restart.
• Centralize full transition-state cleanup and add regression coverage proving state-only cleanup remains audio-stack-free.

Validation

• Core Linux CI passes.
• Linux cannot build the iOS-only Playback module; the new PlaybackTests target is intended for the iOS Simulator/Xcode 26 path.
• Source review confirms recovery now follows the invariant that blend state cannot survive an engine stop.

Slack Thread

  

[sanylax2]

ts better work pt 52000