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/panel.go

@@ -0,0 +1,10 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// Panel is a bordered surface box; its size animates.
+func Panel(c *ui.Context, content func()) ui.Element {
+	t := c.Theme()
+	return ui.Column(c).Gap(t.Space(2)).Padding(t.Space(4)).Background(t.Surface).Border(1, t.Border).
+		Radius(t.Radius).Transition(ui.ElementTransition{Size: true}).Children(content)
+}