Drop dead Method variants, Downloader default fns, parsing/stream_helper unused, suppress_unused leftovers, stale comments

Second pass through the cruft inventory. All deletes — no behavior change.

Method enum + downloader trait:
  * Method::Head / Put / Delete dropped (no caller). Match arms in
    request.rs::as_str() and default_impl.rs::execute() collapse to
    just Get / Post.
  * Request::head builder dropped.
  * Downloader trait's get / get_localized / head / post default
    methods dropped. Every caller went through execute() directly
    anyway; the convenience wrappers carried 4 dead arms each.

Parsing module:
  * bootstrap_visitor_data — pub fn, no caller.
  * discover_web_client_version + CACHED_WEB_CLIENT_VERSION +
    reset_web_client_version_cache — entire sw.js live-version
    discovery pipeline, never wired up by any caller. The cache was
    never populated, so web_client_version() always returned the
    hardcoded constant. Collapsed to just returning the constant.
  * Drops once_cell::Lazy, parking_lot::RwLock around the version
    cache (consent flag still uses RwLock), Regex import, serde_json::Value
    import, downloader/exceptions/Request/InnertubeClientRequestInfo
    imports — all only kept alive by the deleted code.

stream_helper:
  * get_web_embedded_player_response — pub fn, no caller.

js/player_manager + extractor:
  * player_manager::player_hash — pub fn, no caller. Was only kept
    alive by its own definition.
  * extractor::extract_player_hash — pub fn, only called by the now-
    dead player_hash. Test removed alongside.

Stale comments:
  * itag.rs:1 header claimed 53 entries; ITAG_TABLE has 57 and the
    test at line 179 already asserts it.
  * js/mod.rs:12-13 claimed the submodules were 'crate-private
    plumbing' but they're declared pub mod. Tightened the comment to
    explain the integration-test dependency that keeps them public.

Net delete: ~170 LOC of dead surface across 9 files.

src/downloader/default_impl.rs

@@ -54,10 +54,7 @@ impl Downloader for ReqwestDownloader {
     fn execute(&self, request: Request) -> Result<Response, NetworkError> {
         let method = match request.method() {
             Method::Get => reqwest::Method::GET,
-            Method::Head => reqwest::Method::HEAD,
             Method::Post => reqwest::Method::POST,
-            Method::Put => reqwest::Method::PUT,
-            Method::Delete => reqwest::Method::DELETE,
         };
 
         let mut builder = self.client.request(method, request.url());