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/on-usb-role-change.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+# on-usb-role-change.sh — Called by udev on USB role/connect events.
+# Re-detects role, stops/restarts Pd with appropriate config.
+
+set -euo pipefail
+
+ROLE_FILE="/run/usb-role"
+LOG_TAG="[USB-HOTPLUG]"
+
+# Debounce: skip if we just ran (within 3s)
+LOCK="/tmp/usb-role-change.lock"
+if [ -f "$LOCK" ]; then
+    age=$(($(date +%s) - $(stat -c %Y "$LOCK" 2>/dev/null || echo 0)))
+    [ "$age" -lt 3 ] && exit 0
+fi
+touch "$LOCK"
+
+logger -t "$LOG_TAG" "USB role change event — re-detecting..."
+
+/usr/local/bin/detect-usb-role.sh
+
+# Restart Pd if it's running
+if systemctl is-active pd-synth-auto &>/dev/null; then
+    logger -t "$LOG_TAG" "Restarting Pd with new role"
+    systemctl restart pd-synth-auto
+fi