Fix onboard audio: apply ALSA UCM HiFi verb before Pd start

- Add alsaucm -c hw:0 set _verb HiFi to start-synth.sh (onboard mode)
- Add UCM verb to install.sh
- Add UCM verb to justfile fix-ucm recipe
- Without this, the DSP audio routing is not configured and no sound
  reaches the headphone output

scripts/start-synth.sh

@@ -64,6 +64,14 @@ fi
 if [ "$MODE" = "onboard" ] || [ "$MODE" = "i2s" ]; then
     info "Mode: Onboard audio (PMIC headphone / line out on JMISC)"
     AUDIO_OPTS=(-audiooutdev 0 -inchannels 0 -outchannels 1)
+    # Apply ALSA UCM HiFi verb to configure DSP routing (required for audio)
+    if command -v alsaucm &>/dev/null; then
+        alsaucm -c hw:0 set _verb HiFi 2>/dev/null && \
+            info "  UCM HiFi verb applied" || \
+            info "  UCM HiFi verb failed"
+    else
+        info "  alsaucm not found — skipping UCM config"
+    fi
 fi
 
 if [ "$MODE" = "loopback" ]; then