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/helpers_test.go

@@ -0,0 +1,64 @@
+package layout
+
+import (
+	"image"
+	"testing"
+	"time"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/sound"
+	"github.com/egoist/mygo/ui"
+)
+
+func near(img *image.RGBA, x, y int, want ui.Color) bool {
+	p := img.RGBAAt(x, y)
+	d := func(a, b uint8) bool { return a-b < 4 || b-a < 4 }
+	return d(p.R, want.R) && d(p.G, want.G) && d(p.B, want.B)
+}
+
+func lum(img *image.RGBA, x, y int) int {
+	p := img.RGBAAt(x, y)
+	return int(p.R)*2126 + int(p.G)*7152 + int(p.B)*722
+}
+
+func count(cue sound.Cue) int {
+	n := 0
+	for _, c := range sound.Recent() {
+		if c == cue {
+			n++
+		}
+	}
+	return n
+}
+
+func find(t *testing.T, tt *ui.Tester, s string) ui.Rect {
+	t.Helper()
+	r, ok := tt.Find(s)
+	if !ok {
+		t.Fatalf("%q not shown in %q", s, tt.Texts())
+	}
+	return r
+}
+
+// settles checks an animation runs, then stops.
+func settles(t *testing.T, tt *ui.Tester, what string) {
+	t.Helper()
+	if !tt.FramePending() {
+		t.Errorf("%s: no animation", what)
+	}
+	tt.Advance(2 * time.Second)
+	if tt.FramePending() {
+		t.Errorf("%s: animation never settled", what)
+	}
+}
+
+func panics(t *testing.T, what string, fn func()) {
+	t.Helper()
+	defer func() {
+		if recover() == nil {
+			t.Errorf("%s did not panic", what)
+		}
+	}()
+	fn()
+}
+
+func abs(v float32) float32 { return max(v, -v) }