Initial commit: Arduino Uno Q USB Audio Synth project

- USB gadget (UAC1 + MIDI) with boot persistence
- USB host mode with auto-detection
- Pure Data synth patch with ALSA + UDP netreceive MIDI input
- MIDI rawmidi-to-UDP bridge (midi-bridge.py)
- LED matrix visualization (led-matrix-viz.py via arduino-router RPC)
- MCU sketch for LED matrix control (Arduino_RouterBridge + ArduinoGraphics)
- Conftest/OPA Rego policy validation suite for .pd files
- gen_synth_pd.py: programmatic .pd file generator
- System services: midi-bridge, led-matrix-viz, pd-synth-auto, usb-role-detect
- Justfile with deploy, enable, diagnostic recipes
- Diagnostic script (diag.sh)

system/pd-synth-auto.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=Pure Data Synth (auto-detect: gadget or host)
+Documentation=https://github.com/arduino/uno-q-audio-synth
+After=usb-role-detect.service
+Wants=usb-role-detect.service
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/start-synth.sh --auto --verbose
+ExecStop=/usr/bin/killall -s TERM pd
+StandardOutput=journal
+StandardError=journal
+Restart=on-failure
+RestartSec=2
+
+[Install]
+WantedBy=multi-user.target