Tag mapping

How the common tag view picks values from the native tag systems. The table is the one the code reads (TagMappingTable.Rows); this page is generated from it, and a test fails when they differ.

• Rows of one field are tried in table order, and within a row the keys in the order listed. The first value is the field's value; the others stay reachable through CommonTags.Values.
• Native keys are spelled as the system spells them: four-character atom codes for ilst and udta (©nam), reverse-DNS names for key-table metadata (com.apple.quicktime.title), namespace#property for XMP, upper-case names for Matroska (nested SimpleTags as PARENT/CHILD). Matroska names are matched case-insensitively.
• Matroska: only tags that target the file (no track, edition, chapter or attachment UID) feed the view, plus the tags of the only track of a single-track file (except ENCODER, which names the stream's encoder). Within a key the lowest target level of 30 or above comes first, then levels below 30; tags with no language or und come before language-specific ones; tags of the file itself come before tags of the only track.
• 3GPP boxes and QuickTime user-data atoms share four-character codes with different layouts; the reader tells them apart as described in docs/tags.md.
• Sources: the MusicBrainz Picard tag-mapping table (CC0), the ExifTool QuickTime tag documentation (read as tables of facts), the Matroska tag-name list, Apple's QuickTime File Format specification and 3GPP TS 26.244 §8.