Drag a clip onto another to reorder; an S/M/L control changes clip width
(--strip-w), remembered per browser. Trim already existed as the Scene
inspector's length field.
The dragged clip is tracked by its DataTransfer payload, not a closure
variable keyed on DOM position or id -- a redraw mid-drag (any project
change, not just a removal) tears down and rebuilds every cell, and
dragend fires on the original source node regardless of whether a
redraw detached it from the tree. Two adversarial rounds against this
file specifically, both fixed before landing.