youtube: add channel + search continuation (pagination) extractors

The channel Videos tab and search both parsed the page-1 continuation
token but had no way to fetch the next page. Add the next-page fetchers:

- youtube/continuation.rs (new): shared ContinuationPage + the
  appendContinuationItemsAction/continuationItems unwrap + trailing-token
  scan, parameterised on the container key — onResponseReceivedActions
  for browse/channel, onResponseReceivedCommands for search (matches NPE).
- channel.rs: channel_videos_continuation() POSTs browse with
  {continuation: token}; reuse the page-1 rich-grid item parser (factored
  out as parse_rich_grid_item, now also accepting a bare
  lockupViewModel/videoRenderer cell so a layout variant can't silently
  return an empty page while the token keeps advancing).
- search_extractor.rs: search_continuation() POSTs search with
  {continuation: token}; walk itemSectionRenderer contents via the
  existing extract_item_into.

Empty/last page yields a None token so callers stop. New unit tests over
realistic continuation fixtures (bare-lockup, shelf-in-continuation,
last-page, empty body).

src/youtube/mod.rs

@@ -5,6 +5,7 @@
 pub mod channel;
 pub mod client_request;
 pub mod constants;
+pub mod continuation;
 pub mod itag;
 pub mod js;
 pub mod linkhandler;