Graphics: shapes, gradients, and SF Symbols

[colemancda]

Fourth parity tier: graphics. Adds shape views, linear gradients, and SF Symbol images — the biggest visible gap after text/interaction.

What's new

• Shapes — Rectangle, Circle, Capsule, RoundedRectangle(cornerRadius:), each with .fill(Color); emitted as a Shape node the interpreter paints with the matching Compose shape.
• LinearGradient — LinearGradient(colors:startPoint:endPoint:) (and the Gradient form) with UnitPoint; the interpreter picks horizontal/vertical/diagonal from the endpoints and fills available width.
• Image(systemName:) — a curated SF Symbol → Material icon map (~25 common symbols: star, heart, trash, gear, bell, cart, person…), tinted via .foregroundColor and sized via .frame. Unknown symbols and named assets keep the placeholder text so gaps stay visible.

Verification

• swift test — 3 new emission tests (shapes/fill/cornerRadius, gradient colors+endpoints, systemName), 44 total passing
• Emulator (new Graphics screen): all four shapes, horizontal and vertical gradients, and a row of tinted/sized symbols render correctly

Deferred

Ellipse; .stroke; real asset images (no asset pipeline); radial/angular gradients. A shape with only a height (no width) still needs an explicit frame — the fill-the-parent layout behavior isn't modeled (gradients special-case fill-width).