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/detect-i2s-device.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+# detect-i2s-device.sh — Probe for available I2S audio devices
+# and create /run/i2s-audio-dev containing the best candidate.
+# Called by detect-i2s-device.service before pd-synth-i2s starts.
+
+set -euo pipefail
+
+DEVICE_FILE="/run/i2s-audio-dev"
+write_dev() { echo "$1" > "$DEVICE_FILE"; echo "[DETECT] I2S device: $1"; exit 0; }
+
+# Try named I2S devices first (most reliable)
+for dev in "hw:I2S,0" "hw:MI2S,0" "hw:i2s,0" "hw:mi2s,0"; do
+    if aplay -l 2>/dev/null | grep -qi "${dev%,*}"; then
+        write_dev "$dev"
+    fi
+done
+
+# Try probing ALSA device list for anything that looks like I2S
+while read -r card; do
+    id=$(echo "$card" | awk '{print $2}' | tr -d ':')
+    name=$(echo "$card" | cut -d: -f2- | xargs)
+    if echo "$name" | grep -qi -E "(i2s|mi2s|quat|tert|dac|audio)"; then
+        write_dev "hw:$id,0"
+    fi
+done < <(cat /proc/asound/cards 2>/dev/null || true)
+
+# Fallback to hw:0,0 (first audio device, may be HDMI or analog)
+if aplay -l 2>/dev/null | grep -q "^card 0"; then
+    write_dev "hw:0,0"
+fi
+
+# Nothing found
+echo "[DETECT] No I2S audio device found" >&2
+echo "" > "$DEVICE_FILE"
+exit 1