feat: add audio track support with WAV/MP3 playback and looping

Introduce KGAudioTrack and KGAudioRegion as new track/region types
alongside existing MIDI tracks. Users can import WAV, MP3, OGG, FLAC,
and AAC files into audio tracks with full playback, looping, and
solo/mute/volume support.

New core models:
- KGAudioTrack (extends KGTrack with TrackType.Wave)
- KGAudioRegion (extends KGRegion with audio file reference)
- KGAudioPlayerBus (Tone.Gain + ToneBufferSource-based playback)
- KGAudioFileStorage (OPFS media/ directory for binary audio files)

New commands:
- AddAudioTrackCommand (create audio track with player bus)
- ImportAudioCommand (import audio file, create region at playhead,
auto-expand maxBars)

Playback features:
- Audio regions scheduled via Tone.Transport alongside MIDI events
- Loop support with duration capping at loop boundaries
- Resume safety offset to prevent ToneBufferSource timing misses
- Cross-track buffer copy when moving audio regions between tracks
- Proper buffer cleanup on track deletion and project switch

UI changes:
- Separate "+ MIDI" and "+ Audio" buttons in top-left spacer
- Audio track shows upload button instead of instrument selector
- FileImportModal reused for audio file drag-and-drop upload
- Real waveform rendering on audio region canvas
- Green color scheme for audio regions (vs blue for MIDI)
- Pencil icon and resize handles hidden for audio regions
- Cross-type region moves blocked (MIDI ↔ Audio)
- Piano roll blocked for audio regions
- Audio region deletion support

Infrastructure:
- Project structure version bumped to 4 (no-op migration)
- class-transformer discriminators updated for new subtypes
- RemoveTrackCommand updated to clean up audio player buses

src/core/commands/track/AddAudioTrackCommand.ts

@@ -0,0 +1,83 @@
+import { KGCommand } from '../KGCommand';
+import { KGCore } from '../../KGCore';
+import { KGAudioTrack } from '../../track/KGAudioTrack';
+import { KGAudioInterface } from '../../audio-interface/KGAudioInterface';
+import { generateNewTrackName } from '../../../util/miscUtil';
+
+/**
+ * Command to add a new audio track to the project.
+ * Handles both the core model update and audio player bus setup.
+ */
+export class AddAudioTrackCommand extends KGCommand {
+  private trackId: number;
+  private trackName: string;
+  private trackIndex: number;
+  private createdTrack: KGAudioTrack | null = null;
+
+  constructor(trackId?: number, trackName?: string) {
+    super();
+
+    if (trackId === undefined) {
+      const currentProject = KGCore.instance().getCurrentProject();
+      const tracks = currentProject.getTracks();
+      this.trackId = tracks.length > 0
+        ? Math.max(...tracks.map(track => track.getId())) + 1
+        : 1;
+    } else {
+      this.trackId = trackId;
+    }
+
+    this.trackName = trackName || generateNewTrackName();
+    this.trackIndex = 0;
+  }
+
+  execute(): void {
+    const core = KGCore.instance();
+    const currentProject = core.getCurrentProject();
+    const tracks = currentProject.getTracks();
+
+    this.trackIndex = tracks.length;
+
+    this.createdTrack = new KGAudioTrack(this.trackName, this.trackId);
+    this.createdTrack.setTrackIndex(this.trackIndex);
+
+    const updatedTracks = [...tracks, this.createdTrack];
+    currentProject.setTracks(updatedTracks);
+
+    // Create audio player bus for the new track
+    const audioInterface = KGAudioInterface.instance();
+    audioInterface.createTrackAudioPlayerBus(this.trackId.toString());
+
+    console.log(`Added audio track ${this.trackId}`);
+  }
+
+  undo(): void {
+    if (!this.createdTrack) {
+      throw new Error('Cannot undo: no track was created');
+    }
+
+    const core = KGCore.instance();
+    const currentProject = core.getCurrentProject();
+    const tracks = currentProject.getTracks();
+
+    const updatedTracks = tracks.filter(track => track.getId() !== this.trackId);
+    currentProject.setTracks(updatedTracks);
+
+    const audioInterface = KGAudioInterface.instance();
+    audioInterface.removeTrackAudioPlayerBus(this.trackId.toString());
+
+    console.log(`Removed audio track ${this.trackId}`);
+  }
+
+  getDescription(): string {
+    return `Add audio track "${this.trackName}"`;
+  }
+
+  public getTrackId(): number {
+    return this.trackId;
+  }
+
+  public getCreatedTrack(): KGAudioTrack | null {
+    return this.createdTrack;
+  }
+}

src/core/commands/track/RemoveTrackCommand.ts

@@ -39,9 +39,10 @@ export class RemoveTrackCommand extends KGCommand {
       this.originalInstrument = (trackToRemove as KGMidiTrack).getInstrument();
     }
     
-    // Remove audio synth for the track
+    // Remove audio bus for the track (handles both MIDI synth and audio player bus)
     const audioInterface = KGAudioInterface.instance();
     audioInterface.removeTrackSynth(this.trackId.toString());
+    audioInterface.removeTrackAudioPlayerBus(this.trackId.toString());
     
     // Remove the track from the core model
     const updatedTracks = tracks.filter(track => track.getId() !== this.trackId);
@@ -85,11 +86,15 @@ export class RemoveTrackCommand extends KGCommand {
     // Update the core model
     currentProject.setTracks(updatedTracks);
     
-    // Recreate audio synth for the track
+    // Recreate audio bus for the track
     const audioInterface = KGAudioInterface.instance();
-    audioInterface.createTrackSynth(this.trackId.toString(), this.originalInstrument);
-    
-    console.log(`Restored track ${this.trackId} with ${this.originalInstrument} instrument`);
+    if (this.removedTrack.getCurrentType() === 'KGAudioTrack') {
+      audioInterface.createTrackAudioPlayerBus(this.trackId.toString());
+    } else {
+      audioInterface.createTrackSynth(this.trackId.toString(), this.originalInstrument);
+    }
+
+    console.log(`Restored track ${this.trackId}`);
   }
 
   getDescription(): string {