The queue-mutating playback routes (play-now-song, play-next-song,
add-song, play-at) read queue position synchronously, mutate the queue,
then await resolveAndPlay() which suspends at an async URL fetch before
player.play(). With no serialization, two concurrent requests (normal in
login-less guest mode) interleave: the audible song (decided by URL-fetch
latency) can disagree with queue.currentIndex (decided by sync-block
ordering), corrupting "now playing" and causing skipped/duplicate songs.
Add a per-bot async serializer (BotInstance.runExclusive) and wrap the
critical region of all four routes in it. Single-request behavior and
every response shape / validation 400 are preserved; only the critical
region moved inside runExclusive.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
