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/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