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/split_pane.go

@@ -0,0 +1,75 @@
+package layout
+
+import (
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/kit"
+	"git.zacharyzhang.com/ZacharyZhang-NY/CrychicUI/sound"
+	"github.com/egoist/mygo/ui"
+)
+
+const minPane = 40
+
+// SplitPane sets two panes side by side; *size is width.
+func SplitPane(c *ui.Context, size *float32, first, second func()) ui.Element {
+	return split(c, "SplitPane", false, size, first, second)
+}
+
+// split lays out two panes around a draggable handle.
+func split(c *ui.Context, name string, vertical bool, size *float32, first, second func()) ui.Element {
+	if size == nil || !finite(*size) || *size < 0 {
+		panic("layout: " + name + " needs a finite, non-negative size")
+	}
+	t := c.Theme()
+	grip := t.Space(2)
+	var root ui.Element
+	var room float32
+	pane, cursor := func(e ui.Element, v float32) ui.Element { return e.Width(v).ClipX() }, ui.CursorResizeEW
+	if vertical {
+		root = ui.Column(c)
+		room = root.LayoutBounds().H
+		pane, cursor = func(e ui.Element, v float32) ui.Element { return e.Height(v).ClipY() }, ui.CursorResizeNS
+	} else {
+		root = ui.Row(c)
+		room = root.LayoutBounds().W
+	}
+	return root.Fill().AlignItems(ui.Stretch).Children(func() {
+		pane(ui.Column(c), *size).Children(first)
+		handle := ui.Box(c).Cursor(cursor).Center().Label("Divider")
+		if vertical {
+			handle.Height(grip)
+		} else {
+			handle.Width(grip)
+		}
+		dx, dy, moved := handle.Dragged()
+		active := handle.Hovered() || handle.Dragging()
+		color := t.Border
+		if active {
+			color = t.Accent
+		}
+		line := ui.Box(c)
+		if vertical {
+			line.FillWidth().Height(1)
+		} else {
+			line.FillHeight().Width(1)
+		}
+		handle.Children(func() { line.Background(line.AnimateColor("line", color, ui.Snappy)) })
+		if moved {
+			d := dx
+			if vertical {
+				d = dy
+			}
+			next := max(minPane, *size+d)
+			if room > 0 {
+				next = min(next, max(minPane, room-grip-minPane))
+			}
+			if next != *size {
+				*size = next
+				root.SetChanged()
+			}
+		}
+		if _, _, ended := handle.DragEnded(); ended {
+			sound.Play(sound.Tick)
+			kit.Log(name, "resized", "size", *size)
+		}
+		ui.Column(c).Grow(1).Basis(0).Children(second)
+	})
+}