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/holy_grail.go

@@ -0,0 +1,38 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// HolyGrailSlots fill a HolyGrail; only Main is required.
+type HolyGrailSlots struct {
+	Header, Nav, Main, Aside, Footer func()
+}
+
+// HolyGrail lays out header, nav, main, aside and footer.
+func HolyGrail(c *ui.Context, s HolyGrailSlots) ui.Element {
+	if s.Main == nil {
+		panic("layout: HolyGrail needs Main")
+	}
+	t := c.Theme()
+	slot := func(key string, fn func()) ui.Element {
+		e := ui.Column(c).Key(key).Gap(t.Space(2)).Padding(t.Space(3))
+		return e.Transition(ui.ElementTransition{Enter: &ui.Motion{Collapse: true}, Exit: &ui.Motion{Collapse: true}}).
+			Children(fn)
+	}
+	return ui.Column(c).Fill().Children(func() {
+		if s.Header != nil {
+			slot("header", s.Header).Background(t.Surface)
+		}
+		ui.Row(c).Grow(1).AlignItems(ui.Stretch).Children(func() {
+			if s.Nav != nil {
+				slot("nav", s.Nav).Width(t.Rem(12))
+			}
+			slot("main", s.Main).Grow(1).Basis(0)
+			if s.Aside != nil {
+				slot("aside", s.Aside).Width(t.Rem(12))
+			}
+		})
+		if s.Footer != nil {
+			slot("footer", s.Footer).Background(t.Surface)
+		}
+	})
+}