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/scroll_area.go

@@ -0,0 +1,31 @@
+package layout
+
+import (
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"github.com/egoist/mygo/ui"
+)
+
+// ScrollArea scrolls content; edge shadows fade with the offset.
+func ScrollArea(c *ui.Context, content func()) ui.Element {
+	root := ui.Box(c).Clip()
+	st := ui.Local(root, "scroll", func() ui.ScrollState { return ui.ScrollState{} })
+	kit.Settle(c, root, []float32{st.Y, st.MaxY}, func() []float32 { return []float32{st.Y, st.MaxY} })
+	return root.Children(func() {
+		ui.Scroll(c).Fill().TrackScroll(st).Children(content)
+		edge := func(key string, show bool, angle float32) {
+			e := ui.Box(c).Key(key).PassThrough().Absolute().Left(0).Right(0).Height(c.Theme().Space(3))
+			target := float32(0)
+			if show {
+				target = 1
+			}
+			e.Gradient(kit.Shade(0.12), kit.Shade(0), angle).Opacity(e.AnimateTo("o", target, ui.Snappy))
+			if key == "top" {
+				e.Top(0)
+			} else {
+				e.Bottom(0)
+			}
+		}
+		edge("top", st.Y > 0, 180)
+		edge("bottom", st.Y < st.MaxY, 0)
+	})
+}