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/layers.go

@@ -0,0 +1,24 @@
+package layout
+
+import (
+	"time"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"github.com/egoist/mygo/ui"
+)
+
+// Layers stacks layers on one box; each fades in turn.
+func Layers(c *ui.Context, layers []func()) ui.Element {
+	if len(layers) == 0 {
+		panic("layout: Layers needs a layer")
+	}
+	return ui.Box(c).Children(func() {
+		for i, l := range layers {
+			b := ui.Box(c).Key(i)
+			if i > 0 {
+				b.Absolute().Top(0).Right(0).Bottom(0).Left(0)
+			}
+			kit.Arrive(b, ui.Pose{Fade: 1}, time.Duration(i)*80*time.Millisecond).Children(l)
+		}
+	})
+}