add boot-time synth-diagnostic.service

- synth-diagnostic.service: runs test-unit.py before pipeline services
- Type=oneshot, After=prometheus-exporter, Before=pipeline services
- Pipeline services: After=synth-diagnostic
- deploy: restart prometheus-exporter → start synth-diagnostic → restart pipeline
- install.sh: includes synth-diagnostic in SERVICES array

system/led-matrix-viz.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=LED Matrix MIDI Visualization
-After=arduino-router.service
+After=arduino-router.service synth-diagnostic.service
 
 [Service]
 User=arduino

system/midi-bridge.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=MIDI rawmidi → Pd UDP bridge (auto-detects gadget/host)
-After=multi-user.target
+After=multi-user.target synth-diagnostic.service
 
 [Service]
 Type=simple

system/pd-synth-onboard.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Pure Data Synth (Onboard PMIC Audio)
-After=multi-user.target
+After=multi-user.target synth-diagnostic.service
 Wants=midi-bridge.service
 
 [Service]

system/prometheus-exporter.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Prometheus metrics exporter for synth pipeline
-After=multi-user.target
+After=multi-user.target synth-diagnostic.service
 
 [Service]
 Type=simple

system/synth-diagnostic.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=Synth pipeline diagnostic — runs unit tests at boot
+DefaultDependencies=no
+After=multi-user.target prometheus-exporter.service
+Before=midi-bridge.service pd-synth-onboard.service led-matrix-viz.service
+
+[Service]
+Type=oneshot
+ExecStart=/home/arduino/uno-q-audio-synth/scripts/run-tests.sh
+StandardOutput=journal
+StandardError=journal
+RemainAfterExit=yes
+
+# Don't let failures cascade — critical for headless boot
+# On failure: services won't start, board stays SSH-accessible
+FailureAction=none
+
+[Install]
+WantedBy=multi-user.target