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>

gallery/demos.go

@@ -1,3 +1,5 @@
 package gallery
 
-var entries []Entry
+import "slices"
+
+var entries = slices.Concat(layoutEntries)