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/centered.go

@@ -0,0 +1,13 @@
+package layout
+
+import (
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"github.com/egoist/mygo/ui"
+)
+
+// Centered centers content in its room; the content scales in.
+func Centered(c *ui.Context, content func()) ui.Element {
+	return ui.Column(c).Fill().Center().Children(func() {
+		kit.Arrive(ui.Box(c), ui.Pose{Scale: 0.96, Fade: 1}, 0).Children(content)
+	})
+}