Self-review fixups: make "first track" explicit (by disc/track order) and
replace the awkward "so existing tests continue to compile" phrasing with a
clearer description of why newLibraryRouter needs extending.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Covers /api/albums/{id}/cover and /api/tracks/{id}/stream — the byte-serving
endpoints Plan 2's cover_url / stream_url forward references point at.
Duplicates protocol-agnostic helpers locally per subsonic-is-legacy direction
rather than extracting a shared package.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
bvandeusen