aggregation buckets to flat array

Converts aggregation buckets to a flat array of objects.

Recursively walks nested aggregation buckets starting from ONLY the first input element and emits one flat object per leaf bucket, carrying each level's key and doc_count under aggregation-name-prefixed fields plus a joined unique_key. The transpose output aggregates leaf metrics per first-level key (summing count and sum, tracking min and max, rounded avg), sorted by group. Both outputs are empty when the input is empty or its first element is not an object.