Inline fixes:
- Remove TDRC frame from ID3 writer (ID3v2.3 uses TYER only, not TDRC)
- Add try/finally cleanup in worker to prevent VFS leaks on errors
- Fix Blob creation to use Uint8Array directly (avoid extra bytes)
- Replace loadFFmpeg race guard with promise singleton pattern
- Add -map_metadata -1 to strip source metadata (prevent duplicate ID3)
Error handling improvements:
- Create MP3EncodingError class with code property for reliable detection
- Update api.js to use instanceof check instead of string matching
- Pass AbortSignal to encodeToMp3 for proper cancellation support
- Remove error re-wrapping in mp3-encoder.js (preserve original errors)
Technical details:
- Promise singleton ensures FFmpeg loads once even with concurrent calls
- AbortSignal listener properly cleaned up on success/error/abort
- Virtual FS cleanup in finally block prevents file leaks
- MP3EncodingError.code = 'MP3_ENCODING_FAILED' for robust detection
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