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/cluster.go

@@ -0,0 +1,15 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// Cluster wraps items aligned Start, Center or End; items glide.
+func Cluster(c *ui.Context, align ui.Align, items []func()) ui.Element {
+	if align != ui.Start && align != ui.Center && align != ui.End {
+		panic("layout: Cluster align must be Start, Center or End")
+	}
+	return ui.Row(c).FillWidth().Wrap().Justify(align).Gap(c.Theme().Space(2)).Children(func() {
+		for i, it := range items {
+			ui.Box(c).Key(i).Transition(ui.ElementTransition{Position: true}).Children(it)
+		}
+	})
+}