- Updates get_format_string to strictly prioritize `b` (best single file) before falling back to separated tracks (`bestvideo+bestaudio`). This prevents OOM (Killed) errors when merging massive HEVC containers from TikTok.
- Injects a non-blocking `ffprobe` execution inside `download_video` to audit the audio codec. It displays clear warnings on the console if the file is mute or requires extra libraries (`libavcodec-extra`).
- Fixed tests to support the additional `subprocess.run` mock call without failures.