Add virtual 4.1 surround sound configuration for zix790prors

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.

machines/zix790prors/configuration.nix

@@ -7,10 +7,10 @@
 with lib;
 
 {
-  imports =
-    [ # Include the results of the hardware scan.
-      ./hardware-configuration.nix
-    ];
+  imports = [
+    ./hardware-configuration.nix
+    ./virtual-surround.nix
+  ];
 
   roles = {
     audio.enable = true;