Merge K Sorted Log Streams

(1) **Input:** `k` log streams—each stream is a list of `[timestamp, message]` entries sorted by timestamp. (2) **Task:** Merge all streams into one ordered stream by timestamp. (3) **Tie-breaking:** When timestamps are equal, use stable merge—entries from lower-indexed streams first. (4) **Output:** A single list of `[timestamp, message]` in ascending timestamp order. (5) **Data:** Streams may be empty; timestamps are integers, messages are strings.