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/sidebar_layout.go

@@ -0,0 +1,42 @@
+package layout
+
+import (
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/icon"
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/sound"
+	"github.com/egoist/mygo/ui"
+)
+
+// SidebarLayout puts a side column by main; side springs open.
+func SidebarLayout(c *ui.Context, open *bool, side, main func()) ui.Element {
+	if open == nil {
+		panic("layout: SidebarLayout needs open")
+	}
+	t := c.Theme()
+	root := ui.Row(c).Fill().AlignItems(ui.Stretch)
+	return root.Children(func() {
+		col := ui.Column(c).ClipX().Background(t.Surface)
+		w := float32(0)
+		if *open {
+			w = t.Rem(16)
+		}
+		col.Width(col.AnimateTo("w", w, ui.Snappy)).Children(func() {
+			ui.Column(c).Width(t.Rem(16)).Grow(1).Padding(t.Space(3)).Gap(t.Space(1)).Children(side)
+		})
+		ui.Column(c).Grow(1).Basis(0).Children(func() {
+			glyph := "panel-left-open"
+			if *open {
+				glyph = "panel-left-close"
+			}
+			b := ui.Button(c, "").Label("Toggle sidebar").Margin(t.Space(2)).Children(func() {
+				ui.Icon(c, icon.Must(glyph)).Size(16, 16).ContentTransition(ui.ContentTransition{})
+			})
+			if kit.Clicked(kit.Press(b), sound.Toggle) {
+				*open = !*open
+				root.SetChanged()
+				kit.Log("SidebarLayout", "toggled", "open", *open)
+			}
+			ui.Column(c).Grow(1).Children(main)
+		})
+	})
+}