Split screen and picture in picture

Two things on screen means the layout has to be deliberate.

Workflow

1. Stacked (vertical): each clip on its own track, applyLayout "split-top" on one and "split-bottom" on the other: mask, position and cover-fit in one command, no bars.
2. Side by side: each clip updateClip scale about 0.5 with an x offset left and right (origin is the canvas CENTRE, +y is DOWN), setMask rectangle to trim spill.
3. Picture in picture / facecam: the overlay on a later track (it draws on top), updateClip scale about 0.28 into a corner, cornerRadius for softness, clear of captions and the action. Or applyLayout "framed".
4. Audio hierarchy is the whole job: the reactor's voice sits ON TOP. autoDuck the source under the commentary and level the voice (setAudio compress).
5. Captions for the commentary, placed where neither pane is obscured.
6. Keep both panes visible the whole time unless a moment justifies going full screen on one; cut to the reaction only when there IS a reaction.

Gotchas

• Tracks later in the document's track list draw above earlier ones. An overlay on an earlier track is buried under the full-frame clip.
• applyLayout assumes 16:9 media when mediaWidth/mediaHeight are not passed; pass them for portrait sources.
• Check every pane on preview frames: faces must not be cropped by the mask.