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/columns.go

@@ -0,0 +1,16 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// Columns gives each item an equal column; widths animate.
+func Columns(c *ui.Context, items []func()) ui.Element {
+	if len(items) == 0 {
+		panic("layout: Columns needs an item")
+	}
+	return ui.Row(c).FillWidth().Gap(c.Theme().Space(4)).AlignItems(ui.Start).Children(func() {
+		for i, it := range items {
+			ui.Column(c).Key(i).Grow(1).Basis(0).Transition(ui.ElementTransition{Size: true, Position: true}).
+				Children(it)
+		}
+	})
+}