Top k items per category

Given `events`, a list of [category, item, score] triples (category/item are strings, score is an int), return the top `k` items per category by total score. For each category, sum scores per item, then keep the k items with the highest totals; break ties by item name ascending. Output a list of [category, item, total] sorted by category ascending, then total descending, then item ascending. If a category has fewer than k distinct items, return all of them.