Function for_each_diff_pair 

pub fn for_each_diff_pair<B, F>(batch: &B, on_diff_pair: F)
where
    B: BatchReader<Diff = Diff>,
    B::Time: Copy,
    B::ValOwn: 'static,
    F: FnMut(&<B::KeyContainer as BatchContainer>::Owned, B::Time, DiffPair<B::ValOwn>),

Walks batch and invokes on_diff_pair for each DiffPair at each (key, timestamp).

Within a key, diffs are partitioned by sign into retractions (befores) and insertions (afters), sorted by timestamp, and zipped into DiffPairs via a merge-join. Pairs are emitted in ascending timestamp order for a given key; no ordering is guaranteed across keys. Callers are responsible for tracking (key, timestamp) boundaries themselves if they need to detect groups with more than one pair (e.g., for primary-key violation checks).