fix: restore per-batch helper, then remove the need for helpers entirely

[KrX3D]

Two commits — merge both together.

1. Revert the helper cap. It stalled loading:

keep-one.register  BO7rZs_CYQs
keep-one.reused    active:["BO7rZs_CYQs"]
continuation.detected  x1     <- and nothing further, ever

YouTube's refill loop needs each continuation to append at least one item. Hand it an empty batch and it treats the fetch as satisfied and stops asking. The per-batch helper is what keeps that loop alive. Your video-68 test (20% watched vs 80% threshold) proved it — it should have rendered and never did, because batch 5 was never fetched.

2. Serve fully-collected playlists from cache — no helper ever needed.

The initial page response is the one place the append-at-least-one rule doesn't apply. Give it the whole playlist with continuations nulled and there's nothing left to load, so the keep-one branch never runs, no helper is created, and nothing gets stranded in the data model (654577e).

Ordering is the catch: on a first visit the full list only exists after collection finishes. So we collect as now, cache by playlist hash, and reload once — the reloaded initial response is served complete from cache. Revisiting that playlist later in the session hits the cache with no reload at all.

Reloading per batch can't work, for the record: a reload re-fetches the first page, so it loops on batch 1 forever.

Guards: only a complete collection is cached (a partial one would render truncated with no token to load the rest); reload fires at most once per playlist and not if you navigated away; cache bounded to 3 playlists.

Cost: one visible reload the first time you open a playlist. Only active with enablePlaylistBatchCollect on.