Replace the substring signal lists in isLyricsProviderUnavailableError
with typed errors (errLyricsNotFound / errLyricsServiceUnavailable in
the new lyrics_errors.go). Providers now classify failures at the point
of origin: HTTP statuses via lyricsHTTPStatusError (429/5xx cooldown),
netease API codes and paxsenix proxy failures as explicit unavailable,
and every "no lyrics/songs found" path as typed not-found.
Substring matching survives only for error payloads from third-party
proxies (rate limit, missing parameters), whose messages genuinely
arrive as free text.
Two deliberate behavior changes: LyricsPlus 429/5xx responses now cool
the provider down (previously unclassified), and proxy payloads saying
"not found" are treated as not-found instead of disabling the provider.
zarzet