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

install.sh

@@ -3,7 +3,7 @@
 set -euo pipefail
 
 REPO="/home/arduino/uno-q-audio-synth"
-SERVICES=(midi-bridge pd-synth-onboard led-matrix-viz)
+SERVICES=(midi-bridge pd-synth-onboard led-matrix-viz prometheus-exporter)
 
 for svc in "${SERVICES[@]}"; do
     sudo cp "$REPO/system/$svc.service" /etc/systemd/system/