Image carousels become a first-class LFM directive — four variants, and a timestamp convention that turns out to be a run stamp

Screenshots of a setup flow are a sequence, and we had no way to say so in markdown. `:::image-carousel` now does, in four variants split as ImageCarousel--{Variant} siblings. Two findings fell out of building it: the image-prep timestamp records when a file was processed rather than when the screen was captured, and our prose reset out-specifies any component that renders a list inside it.