Insert interval merge

You are given `intervals`, a list of non-overlapping intervals [start, end] sorted ascending by start, and a single `new_interval` [start, end]. Insert the new interval and merge as needed so the result remains sorted and non-overlapping. Return the new list. Two intervals overlap (and merge) if they touch — i.e. [1,2] and [2,3] merge into [1,3]. Up to 10^4 intervals; aim for O(n) with a single pass.