[fix] fix panic for can not search all the vector for not clear the searcher.seen when lower_search

[miamia0]

In the lower_search function, the searcher.seen state is not properly refreshed, which may cause certain entries to be skipped during the search.

    fn lower_search(&self, layer: &[Node<M>], searcher: &mut Searcher<Met::Unit>) {
        searcher.candidates.clear();
        let &Neighbor { index, distance } = searcher.nearest.first().unwrap();
        searcher.nearest.clear();
        >>> searcher.seen.clear();
        let new_index = layer[index].next_node as usize;
        let candidate = Neighbor {
            index: new_index,
            distance,
        };
       >>>  searcher.seen.insert(layer[index].zero_node);
        searcher.nearest.push(candidate);
        searcher.candidates.push(candidate);
    }

[miamia0]

Hi @vadixidav. Just wanted to check if you have a chance to take a look at the PR. I'd be happy to address any feedback or make adjustments if needed.
Thanks for your time and effort in maintaining this project! (｡･∀･)ﾉﾞ