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/container.go

@@ -0,0 +1,22 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// ContainerOptions tune a Container.
+type ContainerOptions struct {
+	// MaxWidth is 64 rem when zero.
+	MaxWidth float32
+}
+
+// Container is a centered, padded page column; its width animates.
+func Container(c *ui.Context, o ContainerOptions, content func()) ui.Element {
+	t := c.Theme()
+	if o.MaxWidth == 0 {
+		o.MaxWidth = t.Rem(64)
+	}
+	if !finite(o.MaxWidth) || o.MaxWidth < 0 {
+		panic("layout: Container MaxWidth must be finite and non-negative")
+	}
+	return ui.Column(c).FillWidth().MaxWidth(o.MaxWidth).Margin(0, ui.Auto).Padding(t.Space(6)).
+		Transition(ui.ElementTransition{Size: true}).Children(content)
+}