Adds the Subsonic browse wire types (MusicFolder, Index, ArtistRef,
ArtistDetail, AlbumRef, AlbumDetail, SongRef, and their response
envelopes) with dual json+xml tags so a single struct serves both
formats. Includes UUID wire helpers (uuidToID/parseUUID), indexing
helpers (indexLetter), MIME mapping (contentTypeForFormat), and the
artist-name resolver used by album listings.
IDs are bare UUID strings on the wire — endpoint context disambiguates.
apiKey (OpenSubsonic) is preferred. t+s uses md5(subsonic_password+salt)
with a constant-time compare. p is disabled by default and gated by
SubsonicConfig.AllowPlaintextPassword; enc:HEX obfuscation decoded.
Auth failures write a Subsonic "failed" envelope so clients don't see
HTTP 401.
Renders one wire format per ?f= parameter; JSON wraps in
{"subsonic-response":...}, XML emits with xmlns, JSONP wraps in callback
or falls back to JSON when callback is empty.
bvandeusen