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/inset.go

@@ -0,0 +1,10 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// Inset is a recessed surface whose color follows the theme.
+func Inset(c *ui.Context, content func()) ui.Element {
+	t := c.Theme()
+	e := ui.Column(c).Gap(t.Space(2)).Padding(t.Space(3)).Radius(t.Radius)
+	return e.Background(e.AnimateColor("bg", t.Surface, ui.Smooth)).Children(content)
+}