Move the cut between two neighbours by delta_ms (positive moves it later): left_id's end and right_id's start move together, and their footage in/out points follow. The pair's total length and every other element stay where they are. The neighbours are a scene and the next scene, or two touching clips on one track, left one first. Refuses, naming the fix: an unknown id, ids that do not share a cut, a locked track, or a delta that would leave either side under 100ms, past its length cap or past its footage (the refusal gives the delta_ms range that fits).