Remove dead 'currentVolume' variable that was never written to, causing
SwitchDevice to always restore volume to 0 after switching audio devices.
- Remove currentVolume from package state
- SwitchDevice now sets mix.MAX_VOLUME after reopening device
- GetVolume128 now queries SDL directly instead of reading stale variable
- Add unit tests for parseSinks, parseSinkLine, and ConvertVolume
- Add changelog.txt tracking all audio bug fixes