Explorer Index

Write createExplorer(records) where each record has at least { id, score }. It RETURNS an object with: - byId(id) — the record with that id, or null (build a Map once, up front) - count() — the number of records - top(n) — the first n records sorted by score, highest first; ties keep their original order; if there are fewer than n, return them all

Difficulty: intermediate