Add layout components 0001-0025

Stack through StickyHeader, with behavior tests and gallery demos.
kit gains Arrive, Settle (paint-time layout check) and Shade.
Several actions return named ui.Element results; mygo vet forbids
elements in structs.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

layout/aspect_box.go

@@ -0,0 +1,12 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// AspectBox holds width over height at ratio; it resizes smoothly.
+func AspectBox(c *ui.Context, ratio float32, content func()) ui.Element {
+	if !finite(ratio) || ratio <= 0 {
+		panic("layout: AspectBox ratio must be finite and positive")
+	}
+	return ui.Box(c).FillWidth().AspectRatio(ratio).Clip().Transition(ui.ElementTransition{Size: true}).
+		Children(content)
+}