feat: adopt constraint-based layout protocol with paint_bounds

[AsPulse]

Switch the component pipeline to a Flutter / SwiftUI-style constraint-based layout protocol, and add the painting primitives needed for effects to work inside layout containers. PR #6's BouncingDot still took scene_width; that's gone.

• Trait surface: view_box() → layout(Constraints) -> Vec2 + paint_bounds(size) -> Rect + render(size [, target]). Constraints { min, max } with tight / loose / tighten_cross lives in tellur-core::geometry alongside Axis.
• Layout containers: SizeMode { Fill, Hug, Fixed(f32) }; Sized / Place / Frame replace Align; CrossAlign::Stretch propagates a tight cross-axis constraint so children fill the stack.
• paint_bounds propagation: raster containers union the child's paint rect into their own. DecoratedBox keeps the default and acts as a natural clip boundary. composite_children takes paint_rect: Rect.
• #[available] attribute on a fn argument lets a component opt in to the parent-assigned size at render time.
• tellur-renderer::shadow (new): DropShadow with a 3-pass box blur; paint_bounds and padding both expand by 3 * radius.
• Examples migrated. BouncingDot drops scene_width and wraps the circle in a DropShadow.