Root cause: OSStatus 561145187 (!rec) - AVAudioSession was not in
.playAndRecord mode when VoiceProcessingIO tried to start.
Two changes:
1. Activate session BEFORE rust.connect() so it's ready when auto-join runs
2. Defensive activation in Connected handler as backup
This fixes both the !rec error and the subsequent 'voice_in already taken'
error that occurred after auto-join consumed voice_in on failure.