Flutter has no inner shadow, so InsetShadowDecoration paints them the way
Chromium does: clip to the box, fill the plane minus the offset-and-shrunk
box, blur the ring. It also takes a gradient fill, outer drop shadows, and a
border, and it tweens from a plain rounded BoxDecoration so animated
containers can fade a selection in.

Settings.raised(color, radius) builds the lit surface every selected or
primary state now shares: a fill lighter at the top, a 1px light inside the
top edge, a 1px shade inside the bottom, a 1px shadow beneath, bare sides.
Two lightness numbers set the lift for every base color. DESIGN.md records
the rule under Tonal First. Pixel tests cover the edges, the gradient, the
drop shadow, and the tween.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>