Sixth tier: appearance modifiers.

What's new

• .border(_:width:), .shadow(radius:…), .clipShape(_:) — fold into the Compose Modifier chain (border, shadow, clip with the shape mapped from Rectangle/Circle/Capsule/RoundedRectangle).
• .overlay(alignment:content:) — an Overlay node whose base and overlay stack in a Box, the overlay sized to the base via matchParentSize and positioned by alignment.

Also fixed

Color now fills the width offered it (SwiftUI semantics) — height-only colors previously collapsed to zero width (visible once an overlay's base was a Color.frame(height:)). fillMaxWidth is applied after the modifier chain so an explicit frame width still constrains. Same correction applied to the gradient path.

Verification

• swift test — 4 new tests (border, clipShape, shadow, overlay base/overlay children + alignment), 52 total passing
• Emulator (new Appearance screen): bordered text, a shadowed card, circle- and capsule-clipped fills, a badge overlay (bottom-trailing), and a full-width bar with a centered overlay label all render correctly

Deferred

.tint (control theming); custom shadow color/offset (radius → elevation only); stroked shapes.