[Feature Request] opus audio support

```
Stream #1:0(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
[mp4 @ 0x1195700] track 1: codec frame size is not set
[mp4 @ 0x1195700] opus in MP4 support is experimental, add '-strict -2' if you want to use it.
Could not write header for output file #0 (incorrect codec parameters ?): Experimental feature
```

No source file, test, or entry point is named. Start by locating the JavaScript code that invokes FFmpeg for MP4 merging and reproduce the shown Opus failure; then identify the project’s expected handling for Opus audio. Done means an Opus audio stream can be merged into MP4 without the reported experimental-feature error.

Written by the indexing model from the issue text.