dixie: fail over USB Wi-Fi to internal CNVi

NM dispatcher plus a boot oneshot keep the AX9L (mt7921u) primary and
bring up the on-box iwlwifi profile if the dongle is gone. Connection
PSKs stay on-box; this only selects the radio.

hw/dixie/default.nix

@@ -128,6 +128,105 @@ let
       TimeoutStartSec = "600";
     };
   };
+
+  # USB AX9L (mt7921u) is primary; internal CNVi (iwlwifi/wlo1) is fallback.
+  # Connection profiles stay on-box (PSK); this only selects which radio is up.
+  usbWifiMac = "74:19:f8:17:5e:53";
+  usbWifiConn = "ATTjkThnDS";
+  internalWifiConn = "ATTjkThnDS-iwlwifi";
+  internalWifiIf = "wlo1";
+  wifiUsbFailover = pkgs.writeShellScript "wifi-usb-failover" ''
+    set -eu
+    nmcli=${pkgs.networkmanager}/bin/nmcli
+    logger=${pkgs.util-linux}/bin/logger
+    systemd_run=${pkgs.systemd}/bin/systemd-run
+    tr=${pkgs.coreutils}/bin/tr
+
+    USB_MAC="${usbWifiMac}"
+    USB_CONN="${usbWifiConn}"
+    FALLBACK_CONN="${internalWifiConn}"
+    INTERNAL_IF="${internalWifiIf}"
+
+    log() { "$logger" -t wifi-usb-failover -- "$*"; }
+
+    iface_mac() {
+      "$tr" 'A-F' 'a-f' < "/sys/class/net/$1/address" 2>/dev/null || true
+    }
+
+    usb_iface() {
+      for n in /sys/class/net/*; do
+        n=''${n##*/}
+        [ "$n" = "lo" ] && continue
+        [ "$(iface_mac "$n")" = "$USB_MAC" ] && { echo "$n"; return 0; }
+      done
+      return 1
+    }
+
+    is_usb_iface() {
+      [ "$(iface_mac "$1")" = "$USB_MAC" ]
+    }
+
+    demote_internal() {
+      "$nmcli" device set "$INTERNAL_IF" autoconnect no >/dev/null 2>&1 || true
+      "$nmcli" connection down "$FALLBACK_CONN" >/dev/null 2>&1 || true
+      "$nmcli" device disconnect "$INTERNAL_IF" >/dev/null 2>&1 || true
+    }
+
+    promote_internal() {
+      if usb_iface >/dev/null 2>&1; then
+        log "USB still present; not failing over"
+        return 0
+      fi
+      log "USB absent; bringing up $FALLBACK_CONN on $INTERNAL_IF"
+      "$nmcli" device set "$INTERNAL_IF" autoconnect yes >/dev/null 2>&1 || true
+      "$nmcli" connection up "$FALLBACK_CONN" ifname "$INTERNAL_IF" >/dev/null 2>&1 || true
+    }
+
+    settle() {
+      i=0
+      while [ "$i" -lt 20 ]; do
+        if usb_iface >/dev/null 2>&1; then
+          u=$(usb_iface)
+          log "settle: USB $u present; demoting internal"
+          demote_internal
+          if ${pkgs.iproute2}/bin/ip -4 -o addr show dev "$u" | ${pkgs.gnugrep}/bin/grep -q ' inet '; then
+            return 0
+          fi
+          "$nmcli" connection up "$USB_CONN" ifname "$u" >/dev/null 2>&1 || true
+          return 0
+        fi
+        i=$((i + 1))
+        sleep 1
+      done
+      log "settle: no USB after 20s; failing over"
+      promote_internal
+    }
+
+    IFACE=''${1-}
+    ACTION=''${2-}
+
+    case "$IFACE" in
+      boot|settle) settle; exit 0 ;;
+      after-usb-down) sleep 3; promote_internal; exit 0 ;;
+    esac
+
+    case "$ACTION" in
+      up)
+        if is_usb_iface "$IFACE"; then
+          log "USB $IFACE up; demoting $INTERNAL_IF"
+          demote_internal
+        fi
+        ;;
+      down)
+        if is_usb_iface "$IFACE"; then
+          log "USB $IFACE down; scheduling failover check"
+          "$systemd_run" --quiet --collect --unit=wifi-usb-failover-down \
+            --property=Type=oneshot "$0" after-usb-down >/dev/null 2>&1 || true
+        fi
+        ;;
+    esac
+    exit 0
+  '';
 in
 {
   imports = [
@@ -203,11 +302,24 @@ in
       upstream = "http://127.0.0.1:11434";
     };
 
-    # Wi-Fi only host. NetworkManager comes from the base config; the AX9L (mt7921u) becomes
+    # Wi-Fi only host. NetworkManager comes from the base config; the AX9L (mt7921u) is
-    # the primary once plugged in and the internal CNVi radio stays as fallback. Connection
+    # primary and the internal CNVi radio is fallback. Connection profiles are copied
-    # profiles are copied from the live-USB session at install time (see the install notes),
+    # from the live-USB session at install time (see the install notes), not stored here.
-    # not stored in this repo.
     networking.networkmanager.wifi.backend = "wpa_supplicant";
+    networking.networkmanager.dispatcherScripts = [
+      { source = wifiUsbFailover; type = "basic"; }
+    ];
+    systemd.services.wifi-usb-failover-boot = {
+      description = "Prefer USB Wi-Fi; fail over to internal CNVi if the dongle is absent";
+      after = [ "NetworkManager.service" ];
+      wants = [ "NetworkManager.service" ];
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+        ExecStart = "${wifiUsbFailover} boot";
+      };
+    };
 
     # Reachable only over the tailnet: 11434 (router), 11433 (vision), 11435 (tether)
     # on tailscale0, and SSH from the LAN (base config opens 22) for bring-up.