QobuzLyricsDocument gains the optional translation member (same content
union as original, new in API 10.0), the content DTOs become
Serialize+PartialEq so the lyrics cache can persist a translation
verbatim, and a lang() accessor exposes the content language. New
fixtures cover embedded translations of each type (wsync/lsync/plain),
the lyricsUrl envelope with translation_requested, empty
translation_langs (feature off), and 9.x documents (no translation).
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