Add Mixer API to Plugin system

Your idea

Add a Mixer API to the Plugin system that exposes programmatic control over mixer settings: volume, balance, mute, solo, sound selection, and MIDI configuration.

Proposed API:

var mixer = part.mixerChannel;

// Audio controls
mixer.volume = 4.0;        // dB range: -60 to +12
mixer.balance = 0.2;       // Pan: -1.0 (left) to 1.0 (right)
mixer.muted = false;
mixer.solo = true;

// Sound selection
var sounds = mixer.availableSounds();
mixer.setSound(sounds[0].id);

// MIDI configuration (for SoundFonts)
mixer.setMidiBank(20);
mixer.setMidiProgram(40);

Problem to be solved

Plugins cannot programmatically configure mixer settings. Users must manually adjust volume, balance, and sound assignments for every part.

Use cases:

• Auto-configure instruments (e.g., detect "bandurria" → assign correct SoundFont preset + optimal volume)
• Apply mixing templates ("Orchestra Balance", "Jazz Combo") with one click
• Batch migrate scores between sound libraries (MS Basic → Muse Sounds)

Prior art

Modern DAWs (Reaper, Logic, Ableton) expose full mixer control to scripting APIs.

Additional context

• Requires async/sync bridge pattern since audio system is async but plugins are sync.
• Should use dB scale for volume to match UI

Checklist

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