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/io/KGAudioFileStorage.ts

@@ -0,0 +1,72 @@
+import { OPFS_CONSTANTS } from '../../constants/coreConstants';
+
+/**
+ * KGAudioFileStorage — Utility for storing and loading audio files
+ * in the OPFS media/ directory alongside project data.
+ */
+export class KGAudioFileStorage {
+  /**
+   * Store an audio file in the project's media/ directory.
+   */
+  public static async storeAudioFile(
+    projectName: string,
+    fileId: string,
+    file: File
+  ): Promise<void> {
+    const mediaDir = await KGAudioFileStorage.getMediaDir(projectName);
+    const fileHandle = await mediaDir.getFileHandle(fileId, { create: true });
+    const writable = await fileHandle.createWritable();
+    await writable.write(await file.arrayBuffer());
+    await writable.close();
+    console.log(`Stored audio file ${fileId} (${file.size} bytes) for project "${projectName}"`);
+  }
+
+  /**
+   * Load an audio file as ArrayBuffer from the project's media/ directory.
+   */
+  public static async loadAudioFile(
+    projectName: string,
+    fileId: string
+  ): Promise<ArrayBuffer> {
+    const mediaDir = await KGAudioFileStorage.getMediaDir(projectName);
+    const fileHandle = await mediaDir.getFileHandle(fileId);
+    const file = await fileHandle.getFile();
+    return file.arrayBuffer();
+  }
+
+  /**
+   * Delete an audio file from the project's media/ directory.
+   */
+  public static async deleteAudioFile(
+    projectName: string,
+    fileId: string
+  ): Promise<void> {
+    try {
+      const mediaDir = await KGAudioFileStorage.getMediaDir(projectName);
+      await mediaDir.removeEntry(fileId);
+      console.log(`Deleted audio file ${fileId} from project "${projectName}"`);
+    } catch (error) {
+      console.warn(`Failed to delete audio file ${fileId}:`, error);
+    }
+  }
+
+  /**
+   * Generate a unique audio file ID preserving the original file extension.
+   */
+  public static generateAudioFileId(originalFileName: string): string {
+    const ext = originalFileName.split('.').pop()?.toLowerCase() || 'wav';
+    const timestamp = Date.now();
+    const random = Math.random().toString(36).substring(2, 10);
+    return `audio_${timestamp}_${random}.${ext}`;
+  }
+
+  /**
+   * Get the media directory handle for a project.
+   */
+  private static async getMediaDir(projectName: string): Promise<FileSystemDirectoryHandle> {
+    const root = await navigator.storage.getDirectory();
+    const projectsDir = await root.getDirectoryHandle(OPFS_CONSTANTS.ROOT_DIR);
+    const projectDir = await projectsDir.getDirectoryHandle(projectName);
+    return projectDir.getDirectoryHandle(OPFS_CONSTANTS.MEDIA_DIR, { create: true });
+  }
+}