stabilize systemd services: fix races, masked flooding, dependencies

- pd-synth: add After=usb-gadget-audio.service midi-bridge.service (was Wants= only, race)
- usb-gadget-audio: add Restart=on-failure After=sysinit.target
- pd-synth-i2s: fix Wants=configure-usb-audio.service → usb-gadget-audio.service
- detect-usb-role.sh: hardcode gadget role (all 4 detection methods always fail, skip 2s sleep)
- arduino-router-serial: masked on board (flooded 2173× restarts looking for nonexistent /dev/ttyGS0)

system/usb-gadget-audio.service

@@ -1,5 +1,6 @@
 [Unit]
 Description=Configure USB Audio Gadget
+After=sysinit.target
 Before=pd-synth.service
 
 [Service]
@@ -7,6 +8,8 @@ Type=oneshot
 ExecStart=/usr/local/bin/configure-usb-audio.sh start
 ExecStop=/usr/local/bin/configure-usb-audio.sh stop
 RemainAfterExit=yes
+Restart=on-failure
+RestartSec=2
 
 [Install]
 WantedBy=multi-user.target