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.

Cargo.toml

@@ -19,6 +19,7 @@ reqwest = { version = "0.12", default-features = false, features = ["rustls-tls-
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 thiserror = "1"
+log = "0.4"
 parking_lot = "0.12"
 url = "2"
 once_cell = "1"