Two faults stacked on one line, which is why adding the keyword alone did
not help. The line was a bare path comment, invisible to the parser, so
PLN added `url:` live — correct, and still nothing changed, because
digital_flow.gif is 17MB against the 8MB still/gif guard. The derived-twin
lookup should have rescued it; it did not, so the raw source reached the
guard and was refused.

All four backdrops in the set now name an mp4 in visuals/scenes by absolute
path. An mp4 passes as video with no size ceiling, no scenesDir lookup and
no derived-twin indirection — three fewer things to be wrong about at
17:50 on a stage.