Add /track/lyricsUrl (signed RPC, method name "tracklyricsUrl") plus
the CDN document fetch through the offline choke point. Wire shapes
were validated against live captures (2026-06-10) and diverge from the
inferred yaml: numeric track_id in the step-1 envelope, CDN document
wrapped in {translation_langs, publishers, writers, original}, "wsync"
word-synced discriminator instead of "lsync" (per-word timestamps),
plain lines as objects, and miss = HTTP 404 error body mapped to a
typed Ok(None) instead of an error. Unit tests deserialize fixture
excerpts of the captures; examples/lyrics_probe.rs is the read-only
live probe used for the capture.
Also expose qbz_credentials::load_oauth_token_from_file so a stale
keyring entry cannot shadow the authoritative encrypted token file.
cc
vicrodh
46c15f507b