Add a singleton ITroubleshootingPlaybackStatusStore (Core, alongside
TroubleshootingNotifier) that records the exit code + speed of the most
recent troubleshooting playback session. A new MediatR notification
handler writes to it on PlaybackTroubleshootingCompletedNotification, and
PrepareTroubleshootingPlaybackHandler resets it when a new session starts
(both the channel and media-item lock paths).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>