Dataframe v2: inline deduped latest logic

[teh-cmc]

It is impossible for Chunk::deduped_latest_at_index to be fast with current Arrow limitations.

This PR works around that by inlining that logic into the dataframe walk directly.

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Deployed docs

Commit	Link
5a94ea9	https://landing-3jqjh0069-rerun.vercel.app/docs