Media / foldkit-media

The model

ImageAsset + Provider + Layout  ->  Picture.derive  ->  attributes a view writes
   facts      URL scheme  how wide                        (no I/O)

One Widths ladder, carried by the provider, decides which widths exist. A Layout picks the ones worth offering:

Layout	Candidates	`sizes`
`Fixed({ width })`	the ladder widths covering 1x and 2x	`${width}px`
`Constrained({ width })`	every width up to the one covering 2x	`(min-width: ${width}px) ${width}px, 100vw`
`FullWidth()`	the whole ladder	`100vw`

A width above the original's own is described with the original's width, and only the first such width is offered: a provider that does not enlarge answers all of them with the same pixels.