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/masonry.go

@@ -0,0 +1,41 @@
+package layout
+
+import (
+	"slices"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"github.com/egoist/mygo/ui"
+)
+
+// Masonry puts each item in the shortest of n columns.
+func Masonry(c *ui.Context, columns int, items []func()) ui.Element {
+	if columns < 1 {
+		panic("layout: Masonry needs a column")
+	}
+	gap := c.Theme().Space(3)
+	root := ui.Box(c).FillWidth()
+	w := root.LayoutBounds().W
+	colW := max(0, (w-gap*float32(columns-1))/float32(columns))
+	used, boxes, sum := make([]float32, len(items)), make([]ui.Element, len(items)), make([]float32, columns)
+	root.Children(func() {
+		for i, item := range items {
+			b := ui.Box(c).Key(i).Absolute().Width(colW).Transition(ui.ElementTransition{Position: true})
+			used[i] = b.LayoutBounds().H
+			j := slices.Index(sum, slices.Min(sum))
+			b.Left(float32(j) * (colW + gap)).Top(sum[j]).Children(item)
+			sum[j] += used[i] + gap
+			boxes[i] = b
+		}
+	})
+	if len(items) > 0 {
+		root.Height(slices.Max(sum) - gap)
+	}
+	kit.Settle(c, root, append(used, w), func() []float32 {
+		h := make([]float32, 0, len(boxes)+1)
+		for _, b := range boxes {
+			h = append(h, b.LayoutBounds().H)
+		}
+		return append(h, root.LayoutBounds().W)
+	})
+	return root
+}