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>

gallery/demos_layout_test.go

@@ -0,0 +1,21 @@
+package gallery
+
+import (
+	"testing"
+	"time"
+)
+
+func TestScrollerDemoLogsArrows(t *testing.T) {
+	e := Entries()[23]
+	if e.Name != "Scroller" {
+		t.Fatalf("entry 24 is %s", e.Name)
+	}
+	_, tt := open(e.Path())
+	tt.Advance(time.Second)
+	if err := tt.Click("Scroll forward"); err != nil {
+		t.Fatal(err)
+	}
+	if !tt.HasText("Scroller: forward") {
+		t.Errorf("log strip lacks the event: %q", tt.Texts())
+	}
+}