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/auto_grid.go

@@ -0,0 +1,22 @@
+package layout
+
+import (
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"github.com/egoist/mygo/ui"
+)
+
+// AutoGrid fits as many columns as minWidth allows.
+func AutoGrid(c *ui.Context, minWidth float32, items []func()) ui.Element {
+	if !finite(minWidth) || minWidth <= 0 {
+		panic("layout: AutoGrid minWidth must be finite and positive")
+	}
+	gap := c.Theme().Space(3)
+	root := ui.Grid(c).FillWidth().Gap(gap)
+	w := root.LayoutBounds().W
+	kit.Settle(c, root, []float32{w}, func() []float32 { return []float32{root.LayoutBounds().W} })
+	return root.Columns(max(1, int((w+gap)/(minWidth+gap)))).Children(func() {
+		for i, it := range items {
+			ui.Box(c).Key(i).Transition(ui.ElementTransition{Position: true, Size: true}).Children(it)
+		}
+	})
+}