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/check.go

@@ -0,0 +1,6 @@
+package layout
+
+import "math"
+
+// finite reports v is neither NaN nor infinite.
+func finite(v float32) bool { return !math.IsNaN(float64(v)) && !math.IsInf(float64(v), 0) }