perf: build the full-playlist cache from YouTube's own batches, not a re-download

[KrX3D]

The cache works — but almost the entire wait before it was self-inflicted:

19:50:42.395 .. 19:50:43.128   YouTube delivers every batch     0.73s
19:50:45.006 .. 19:50:52.963   our collector re-fetches them    8.0s
19:50:52.966   full_cache.stored -> reloading
19:50:53.135   full_cache.injected

YouTube had the whole playlist at 43.128. The collector then spent 8s re-downloading exactly that at 2.5s per request before the reload could happen — so the helpers sat on screen ~10s instead of ~1s.

adblock.js already intercepts every continuation response before filtering, so the raw items are taken straight from there. When a response arrives with no continuation token the playlist is complete: cache and reload immediately, no extra network work.

The hook sits before filterContinuationItems deliberately — after it, an all-watched batch is reduced to the single kept helper.

Also cancels the background collector once the native path completes (flag cleared on navigation and at each collection start, so a finished playlist can't block the next one). The collector is still kept for playlists that aren't fully watched, where YouTube only fetches on scroll and the prefetch still does real work.

Expected: full_cache.from_native → stored → reloading → injected about a second after opening, with batch_collect.cancelled instead of eight seconds of batch_fetched.