Elevation (docs/design-language.md, “Surfaces”): a window tile casts
one soft shadow that lifts it off the desktop ground; a floating thing
casts two, a tight contact layer that defines its edge and a wide one
that reads as distance. Bars, cards and selected controls cast none: a
selection is a lift (crate::tokens::Intents::selection).
How much of the dark material’s floating shadow a light material
keeps: the design language’s light floating layer (alpha 0.14) over the
dark one (0.38).