Vorbis comments, FLAC PICTURE blocks and METADATA_BLOCK_PICTURE

RecursiveAI.OneMedia.Tags.VorbisComment: VorbisCommentReader, VorbisCommentBlock (an ITagContainer, system vorbis), PictureBlockHeader.

• Layout (Vorbis comment specification §Structure): vendor string (32-bit little-endian length, UTF-8), a 32-bit count, then that many length + "NAME=value" strings. Ogg Vorbis and the Theora, Speex comment headers end with a framing bit; Opus and FLAC do not. Names are 0x20–0x7D without = and case-insensitive; values are UTF-8 (invalid sequences are replaced and reported). The vendor string is NativeTag vendor and VorbisCommentBlock.Vendor. A length past the end of the block, a count past the available bytes and a name without = are typed defects (vorbis.truncated, vorbis.vendor-size, vorbis.field-size, vorbis.count, vorbis.no-equals, vorbis.name, vorbis.trailing; pictures: vorbis.picture, vorbis.picture-base64, vorbis.picture-too-large); the comments read so far are kept.
• Ranges. Each tag has the range of its length + string item and of its value. In an Ogg stream the offsets are relative to the start of the comment packet, because a packet can span pages; the container's Range is the physical page span. In native FLAC the offsets are file offsets.
• Pictures. METADATA_BLOCK_PICTURE holds a base64 FLAC PICTURE block (RFC 9639 §8.8): picture type (32 bits), MIME, description, width, height, depth, colours, data. Each becomes an image tag (type, MIME, description) whose value is decoded when read. A FLAC PICTURE block gives the same tag directly. Cover pictures reach CommonTags.Pictures with their PictureType.
• Common view. TagMappingTable rows for the Vorbis names (TITLE, ARTIST/PERFORMER, ALBUM, ALBUMARTIST, COMPOSER, GENRE, DATE, TRACKNUMBER, TRACKTOTAL/TOTALTRACKS, DISCNUMBER, COMMENT, DESCRIPTION, COPYRIGHT, LYRICS, ENCODER, …), matched case-insensitively. TRACKNUMBER=3/12 gives track 3 and total 12. REPLAYGAIN_* and R128_* stay native. Facts: DATE is a capture-time candidate (a year or an ISO 8601 date, at the rank of a day-level tag) and ENCODER a software candidate.
• Sources: Xiph Vorbis comment specification, RFC 9639, the Picard tag map (CC0) as a table of names.

Agreement with other readers

Checked on 72 Ogg/Opus/FLAC/Ogg Theora files (including the tag variants written by ffmpeg): common view 141 fields equal and 3 partly equal against ExifTool 13.60 and MediaInfo 26.10, native tags 161 equal. The differences, all explained:

• Duration. ExifTool estimates a Vorbis duration from the nominal bitrate (0.8–6.9 s for 4–596 s files) and gives none for Opus; MediaInfo counts an Opus stream from the granule, so it includes the pre-skip (4.007 s against our 4.000 s, which follows RFC 7845 §5.1).
• Comments of a Theora stream. theora-*.ogv carry a comment header on the video stream; neither tool reports it, we do (vorbis:encoder).
• ATL.NET 7.19 (atl command): same on 121 fields. ATL.NET fills a missing title with the file name (the "ours-empty" lines of the first run are those titles); it does not read PERFORMER as the artist (we do, as the Vorbis comment spec names it the performing artist); it trims values and ours keep the stored text (compared trimmed).