Add assertion for duplicate USB device assignments across VMs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/assertions.nix

@@ -182,6 +182,24 @@ in
           ) effectiveSharedDirs
         )
       ) (lib.attrValues cfg.nixosVms)
+      # USB device assertions: no duplicate VID:PID+serial across VMs
+      ++ (
+        let
+          allUsbDeviceKeys = lib.concatMap (
+            vm:
+            map (
+              dev:
+              "${dev.vendorId}:${dev.productId}:${toString (dev.serial or "")}"
+            ) vm.usbDevices
+          ) (lib.attrValues cfg.nixosVms);
+        in
+        [
+          {
+            assertion = lib.length allUsbDeviceKeys == lib.length (lib.unique allUsbDeviceKeys);
+            message = "USB devices (VID:PID+serial) cannot be assigned to multiple VMs";
+          }
+        ]
+      )
       # netvm: network.netvm must reference a VM with isNetvm = true (skip for "host")
       ++ lib.concatMap (
         vm: