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_stack.go

@@ -0,0 +1,8 @@
+package layout
+
+import "github.com/egoist/mygo/ui"
+
+// SplitStack holds two panes stacked; *size is the first's height.
+func SplitStack(c *ui.Context, size *float32, first, second func()) ui.Element {
+	return split(c, "SplitStack", true, size, first, second)
+}