Prevent crashes when Live Activities reconfigure audio

[sanylax0]

Root cause

Live Activity, Dynamic Island, Siri, and competing media updates can report an audio configuration change while AVAudioEngine.isRunning is still true, then stop the engine asynchronously. That bypassed recovery and left invalid player-node schedules reachable. Quick restarts could also leave a pre-restart lastRenderTime less than one second old, incorrectly authorizing AVAudioPlayerNode.play(at:) on the new engine epoch.

Fix

• Recheck apparently-spurious configuration changes and recover if the engine stops after the callback.
• Make the 20 Hz playback loop recover before touching nodes on a stopped engine.
• Tie synchronized stem starts to the current engine-start host-time boundary.
• Guard stop/seek operations and rebuild invalid schedules on automatic, paused, remote, and scrubber resume paths.
• Cancel paused crossfades whose incoming schedule was invalidated.
• Add regression tests for old-epoch, future, and expired render clocks.

Validation

• Core Linux CI passes.
• Source/actor/lifecycle review completed for interruption, transition, pause/resume, seek, and repeated-restart paths.
• iOS Playback tests require Xcode 26 and cannot run on the Linux agent.

Slack Thread