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

justfile

@@ -104,7 +104,7 @@ deploy:
         cd {{REPO_DIR}} && git pull && \
         chmod +x scripts/*.py scripts/*.sh 2>/dev/null; \
         echo '{{BOARD_PASS}}' | sudo -S systemctl daemon-reload && \
-        echo '{{BOARD_PASS}}' | sudo -S systemctl restart midi-bridge pd-synth-onboard led-matrix-viz && \
+        echo '{{BOARD_PASS}}' | sudo -S systemctl restart midi-bridge pd-synth-onboard led-matrix-viz prometheus-exporter && \
         echo '[OK] Deployed + restarted'"
 
 # Start synth (onboard audio)