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/section.go

@@ -0,0 +1,20 @@
+package layout
+
+import (
+	"time"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"github.com/egoist/mygo/ui"
+)
+
+// Section is a muted heading, growing divider and body.
+func Section(c *ui.Context, title string, content func()) ui.Element {
+	t := c.Theme()
+	return ui.Column(c).Gap(t.Space(2)).Children(func() {
+		kit.Muted(c, title).Bold().FontSize(t.Rem(0.85))
+		line := ui.Box(c).FillWidth().Height(1).Background(t.Border).TransformOrigin(0, 0.5)
+		grow := line.Keyframes("grow", []ui.Keyframe{{Value: 0}, {At: 200 * time.Millisecond, Value: 1, Ease: ui.EaseOut}})
+		line.Scale(grow, 1)
+		ui.Column(c).Gap(t.Space(2)).Children(content)
+	})
+}