Merge two interval lists into a union

You are given two lists of closed intervals [start, end]. Each list is individually sorted by start and contains non-overlapping intervals. Merge the two lists into a single list representing the union of all intervals, returning a minimal set of non-overlapping intervals sorted by start. Specify how you handle touching boundaries (e.g., [1,3] and [3,5]), empty inputs, and invalid intervals (start > end). State the time and space complexity and outline an O(m + n) two-pointer approach.