Implement `merge_intervals(intervals)` for closed intervals. Return disjoint closed intervals that cover exactly the same points, sorted by start. Intervals sharing an endpoint overlap and must be merged. Do not mutate the input. ### Constraints - `0 <= len(intervals) <= 200000` - Each interval is `[start, end]` with `-10^9 <= start <= end <= 10^9`. - Input may be unsorted and may contain duplicates. ### Examples - `[[1,3], [2,6], [8,10], [15,18]]` returns `[[1,6], [8,10], [15,18]]`. - `[[1,4], [4,5]]` returns `[[1,5]]` because the intervals are closed. - `[]` returns `[]`. ```hint Exercise closed-boundary behavior Use intervals that share exactly one endpoint and verify that the shared point is not split across two outputs. ``` ```hint Check representation side effects Include duplicate intervals and verify both the canonical output and the requirement that the input remain unchanged. ```