Create a PipeWire virtual surround sink that routes audio to multiple
physical outputs:
- FL/FR channels → AmazonBasics USB speaker
- RL/RR channels → Fosi BT20A PRO Bluetooth speaker
- LFE channel → AmazonBasics (duplicated to both channels)
Uses loopback modules with systemd services to maintain correct routing,
as PipeWire's target.object parameter doesn't auto-connect properly.
A timer checks every 10 seconds and fixes incorrect connections.
Configuration is machine-specific and isolated in virtual-surround.nix.
