feat: route arrangement playback through mixer

[britle20]

Summary

Connects the arrangement mixer panel to real SONG playback routing.

Closes #43

Changes

• Added serializable track and master mixer state helpers under src/model/mixer.ts.
• Added track-aware arrangement playback events by preserving trackId during clip-instance expansion.
• Routed track-aware scheduled sources through per-track gain/analyser nodes and a master gain/analyser node in the browser audio engine.
• Reworked the mixer panel into a controlled UI for track volume, mute, solo, master volume, and runtime meter display.
• Added unit tests for dB-to-gain conversion, mute/solo effective audibility, mixer state updates, and track-aware event expansion.
• Updated mixer-related docs and marked the feature spec as in review.

Testing

• npm run typecheck
• npm run lint
• npm run test
• npm run build

Manual check:

• npm run dev -- --host 127.0.0.1 --port 5173 started successfully; Vite used http://127.0.0.1:5174/ because 5173 was occupied.
• curl -I http://127.0.0.1:5174/ returned HTTP 200.

Screenshots

If UI changed, add screenshots or a short recording.

Notes / Limitations

• Manual browser audio checks still need reviewer confirmation: place clips on multiple tracks, play in SONG mode, then test fader, mute, solo, master volume, and meters.
• Effect slots remain disabled placeholders.
• Mixer routing applies to track-aware SONG arrangement events; PAT and preview playback remain on the existing direct playback path.

Follow-up

• Future features can persist mixer state once IndexedDB/project persistence is implemented.
• Pan, sends, buses, automation, effects, and export rendering remain separate issues.