Lengthen a song section without breaking sync downstream

To give a video segment more time without changing any other cut, find the chord cycle in an instrumental break with beat-synchronous chroma (top-2 pitch classes per beat shows the period, here 16 beats). Repeat the 16 beats that end exactly at the insertion point (e.g., the vocal entry), so the end of each copy flows into the original audio. Snap cut points to onset peaks within 25 ms and add 12 ms equal-power crossfades. Check that re-tracked inter-beat intervals and spectral flux at the joins sit in the normal beat range. In Remotion, remap song time to frames as t + (t >= INSERT_AT ? INSERT_LEN : 0). Watch boundaries that sit right at the insert point: they must use INSERT_AT - epsilon, or they jump to after the insert.