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/scroller.go

@@ -0,0 +1,38 @@
+package layout
+
+import (
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/icon"
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/sound"
+	"github.com/egoist/mygo/ui"
+)
+
+// Scroller scrolls sideways; arrows move it a page.
+func Scroller(c *ui.Context, content func()) (root, back, forward ui.Element) {
+	t := c.Theme()
+	root = ui.Row(c).FillWidth().Gap(t.Space(2)).AlignItems(ui.Center)
+	st := ui.Local(root, "scroll", func() ui.ScrollState { return ui.ScrollState{} })
+	kit.Settle(c, root, []float32{st.X, st.MaxX}, func() []float32 { return []float32{st.X, st.MaxX} })
+	root.Children(func() {
+		arrow := func(glyph, label string, off bool) ui.Element {
+			b := ui.Button(c, "").Label(label).Disabled(off).Children(func() {
+				ui.Icon(c, icon.Must(glyph)).Size(16, 16)
+			})
+			return kit.Press(b)
+		}
+		back = arrow("chevron-left", "Scroll back", st.X <= 0)
+		view := ui.ScrollHorizontal(c).Grow(1).Basis(0).TrackScroll(st)
+		view.Children(func() { ui.Row(c).Gap(t.Space(3)).Children(content) })
+		forward = arrow("chevron-right", "Scroll forward", st.X >= st.MaxX)
+		page := view.LayoutBounds().W
+		if kit.Clicked(back, sound.Tap) {
+			st.ScrollTo(max(0, st.X-page), 0, ui.Smooth)
+			kit.Log("Scroller", "back", "x", st.X)
+		}
+		if kit.Clicked(forward, sound.Tap) {
+			st.ScrollTo(min(st.MaxX, st.X+page), 0, ui.Smooth)
+			kit.Log("Scroller", "forward", "x", st.X)
+		}
+	})
+	return root, back, forward
+}