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_test.go

@@ -0,0 +1,100 @@
+package layout
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/sound"
+	"github.com/egoist/mygo/ui"
+)
+
+const time2s = 2 * time.Second
+
+func lines(c *ui.Context, n int) func() {
+	return func() {
+		for i := range n {
+			ui.Text(c, fmt.Sprintf("Line %d", i)).Padding(4)
+		}
+	}
+}
+
+func TestScrollAreaEdges(t *testing.T) {
+	tt := ui.NewTester(func(c *ui.Context) {
+		ScrollArea(c, lines(c, 60)).Size(300, 200)
+	}, 300, 200)
+	tt.Advance(time2s)
+	top, bottom := lum(tt.Image(), 290, 1), lum(tt.Image(), 290, 198)
+	if bottom >= top {
+		t.Fatal("no bottom shadow while content continues")
+	}
+	tt.Scroll(150, 100, 0, 200)
+	tt.Advance(time2s)
+	if lum(tt.Image(), 290, 1) >= top {
+		t.Error("no top shadow once scrolled")
+	}
+}
+
+func TestScrollerPages(t *testing.T) {
+	forward := 0
+	tt := ui.NewTester(func(c *ui.Context) {
+		_, _, fwd := Scroller(c, func() {
+			for i := range 12 {
+				ui.Column(c).Size(90, 40).Children(func() { ui.Text(c, fmt.Sprintf("Tile %d", i)) })
+			}
+		})
+		if fwd.Clicked() {
+			forward++
+		}
+	}, 400, 100)
+	taps := count(sound.Tap)
+	if err := tt.Click("Scroll back"); err != nil {
+		t.Fatal(err)
+	}
+	if count(sound.Tap) != taps {
+		t.Error("disabled back arrow played a cue")
+	}
+	x5 := find(t, tt, "Tile 5").X
+	if err := tt.Click("Scroll forward"); err != nil {
+		t.Fatal(err)
+	}
+	tt.Advance(time2s)
+	if count(sound.Tap) != taps+1 || forward != 1 {
+		t.Errorf("forward arrow: %d cues, caller saw %d clicks", count(sound.Tap)-taps, forward)
+	}
+	if x := find(t, tt, "Tile 5").X; x > x5-200 {
+		t.Errorf("Tile 5 moved from %v to %v, less than a page", x5, x)
+	}
+}
+
+func TestStickyHeaderStays(t *testing.T) {
+	tt := ui.NewTester(func(c *ui.Context) {
+		StickyHeader(c, func() { ui.Text(c, "Head") }, lines(c, 60)).Size(300, 200)
+	}, 300, 200)
+	tt.Advance(time2s)
+	y, below := find(t, tt, "Head").Y, find(t, tt, "Line 0")
+	shade := lum(tt.Image(), 290, int(below.Y)+2)
+	tt.Scroll(150, 150, 0, 300)
+	tt.Advance(time2s)
+	if find(t, tt, "Head").Y != y {
+		t.Error("header moved with the scroll")
+	}
+	if lum(tt.Image(), 290, int(below.Y)+2) >= shade {
+		t.Error("no header shadow once scrolled")
+	}
+}
+
+func TestScrollAreaEdgeFollowsContent(t *testing.T) {
+	n := 3
+	tt := ui.NewTester(func(c *ui.Context) {
+		ScrollArea(c, lines(c, n)).Size(300, 200)
+	}, 300, 200)
+	tt.Advance(time2s)
+	flat := lum(tt.Image(), 290, 198)
+	n = 60
+	tt.Frame()
+	tt.Advance(time2s)
+	if lum(tt.Image(), 290, 198) >= flat {
+		t.Error("no bottom shadow once content overflows")
+	}
+}