clitic/vsd ? reverse-engineered prompt

Build me a Rust command line tool and library that can download video streams from DASH and HLS links, including playlists with separate video, audio, and subtitle tracks. I want it to handle protected streams too, so it can work with DRM when keys are available, and it should be able to grab subtitles from fragmented mp4 streams. If ffmpeg is installed, it should merge everything into one playable file at the end, but it should still work without ffmpeg for basic downloading.

Make the download process fast and reliable, with multiple segments fetched at the same time and a clean progress display. It should feel easy to use from the terminal, with a main save command that takes a playlist URL and an output file name. If it makes sense, also include a way to inspect or capture stream info from a browser source. Keep the code clean enough that it can be used as a library in other Rust projects too, and look up current docs online if you need to.