add prometheus exporter + test suite

- prometheus-exporter.py: stdlib-only HTTP metrics on :9090/metrics
- Exposes: ALSA status, hw_ptr rate, service health, mixer controls
- MIDI counters: notes_on, notes_off, errors
- test-unit.py: 10-test suite covering full MIDI→Pd→Audio pipeline
- test-tone.pd, test-midi-log.pd: minimal debug patches
- systemd service for exporter
- install.sh/deploy updated to include exporter

system/prometheus-exporter.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Prometheus metrics exporter for synth pipeline
+After=multi-user.target
+
+[Service]
+Type=simple
+ExecStart=/home/arduino/uno-q-audio-synth/scripts/prometheus-exporter.py
+User=root
+Restart=on-failure
+RestartSec=5
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=multi-user.target