youtube: instrument the extractor with diagnostic logging

The crate had zero logging — every failure, fallback, cache reset, and
empty-parse was silent by construction, which made the 2026 bot-wall outage
nearly undiagnosable. Add `log = "0.4"` and 52 statements across the extractor
following a one-INFO-per-outcome / WARN-on-every-failure / DEBUG-for-internals
policy, so a future break shows up in a log grep instead of a debugger.

Highlights: nsig->throttled-URL fallback (aggregated one WARN per extraction,
never per-format, via a threaded UrlProcessStats counter); the nsig
identity-output trap now returns DeobfError::NsigIdentity and is NOT cached
(previously it cached and permanently throttled); player.js build/eval/install/
StillBad lifecycle; the ANDROID->VISIONOS cascade outcome; visitorData decline +
reset; HTTP 429 bot-flag; channel/search layout-change empties; per-request
DEBUG (query stripped). Plus a `BotDetected` error variant (Display byte-
identical to the old string) so the wall is greppable.

No secrets: only videoId/channel ids, error Displays (already URL/token-scrubbed
at the exceptions.rs choke points), query-stripped endpoints/player.js URLs,
counts, and lengths are logged — never token/poToken/visitorData/signature
values or response bodies.

src/youtube/js/mod.rs

@@ -60,6 +60,8 @@ pub enum DeobfError {
     JsRuntimeFailed(String),
     #[error("nsig output was empty (function neutered?)")]
     NsigEmpty,
+    #[error("nsig output equalled its input (identity — deobfuscator neutered?)")]
+    NsigIdentity,
     #[error("downloader not initialized")]
     DownloaderMissing,
 }