ok, I've spent 15 minutes on it and it doesn't work
first, Matroska does support rotation metadata, but it's retarded: https://matroska.org/technical/specs/index.html#ProjectionPoseYaw
it's 3 different parameters for 3 axis
neverthless, I've managed to create video with correct metadata via mkvmerge
but turns out wm4 added support to mkv's rotattion metadata only to its fork: https://github.com/wm4/mpv_backup/commit/028faacff56d7f90f6b119b0b0ac686fd614825f
there's no such commit in official branch
and even if it was, it won't rotate video as far as I can understand, it only parses it
and even it it was, you would still need to use mpv's own demuxer, because ffmpeg doesn't(?) parse that metadata
and even if it was, Chrome doesn't support that metadata either
fuck this shit