Mark 3D from REQ-VIDEO-LAYOUT, DV profile 20 codecs or an lhvC box; mux with MP4Box, decrypt with mp4decrypt, skip FFmpeg repack, sample cut and hybrid; add {3d} and env MP4Box.
A title with no language now takes the language of an original-language track, else a
video track, so an unlabelled MPD re-parses and -l orig works. A track that no longer
matches its manifest is dropped with a warning instead of failing at download.
The importer builds subtitle tracks from track rows that carry a url, which the format promised and no reader did. The writer keeps a conflicting KID instead of overwriting it, identifies a manifest by its whole URL, writes subtitle rows as srt or vtt rather than enum names, and survives an export file it cannot read.
Only fall back to Title.language when the manifest labels nothing, and reject und as firmly as a missing tag. Exported direct-URL subtitles now replace the re-parsed set, and fetch failures stay distinct from parse failures.
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