Upgrade MyGo to v0.5.5

MyGo v0.5.5 returns ui.Element as a checked value handle instead of a
pointer. Migrated with `mygo migrate-ui`, then fixed what the codemod
leaves to review:

- Element nil checks become .Valid(); absent elements are ui.Element{}.
- Input handlers and paint callbacks run outside the build pass, where
  elements expire and Context methods are inert: read IsDisabled into a
  local during the build, keep a ui.Handle for Focus, use c.Services()
  for clipboard access, create ui.Local state during the build.
- Add core.AutoFocus: v0.5.5 records an overlay's opener after its
  content builds, so same-frame autofocus lost focus restore on close.
  It focuses one frame later; cascader rows defer their focus likewise.
- core.OverlayLayer identifies build passes by the root element, not
  c.Now(), which repeats across passes and under the Tester clock.
- Context menus survive the opening press counted as PressedOutside;
  split button tooltip no longer reorders its open menu; disabled
  menubar rows pass presses to the panel; Form defers blur-started
  async checks so the growing message no longer moves Submit away
  from the release; Combobox drops its disabled copy now that MyGo
  blocks typing under a disabled wrapper (canary flipped to a guard).
- The Tester clock is virtual: tests use Advance/Now instead of
  time.Sleep/time.Now and read element state inside the view. The web
  mirror advances the clock to wall time before each frame and input.
- AGENTS.md and docs describe the value Element API and the new pin.

Snapshot goldens are darwin-only and were not re-verified here.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

chart/network_graph.go

@@ -150,7 +150,7 @@ func NetworkGraph(c *ui.Context, s *NetworkGraphState, nodes []GraphNode, edges
 		})
 	}
 	res, plot := ch.Build(spec)
-	if plot == nil || plot.IsDisabled() {
+	if !plot.Valid() || plot.IsDisabled() {
 		return res
 	}
 	hit := func(x, y float32) string {
@@ -165,7 +165,7 @@ func NetworkGraph(c *ui.Context, s *NetworkGraphState, nodes []GraphNode, edges
 }
 
 // networkInput drags, pans, zooms and moves the focused node.
-func networkInput(plot *ui.Element, s *NetworkGraphState, focus int, nodes []GraphNode, unit func(ui.Rect) float64, hit func(x, y float32) string) {
+func networkInput(plot ui.Element, s *NetworkGraphState, focus int, nodes []GraphNode, unit func(ui.Rect) float64, hit func(x, y float32) string) {
 	b := plot.Bounds()
 	r := ui.Rect{W: b.W, H: b.H}
 	move := func(id string, dx, dy float32) {
@@ -187,8 +187,9 @@ func networkInput(plot *ui.Element, s *NetworkGraphState, focus int, nodes []Gra
 		s.dragging = false
 		slog.Debug("mujica: network_graph drag", "node", s.drag)
 	}
+	disabled := plot.IsDisabled()
 	plot.HandleInput(func(ev ui.InputEvent) bool {
-		if ev.Kind != ui.InputScroll || ev.DY == 0 || plot.IsDisabled() {
+		if ev.Kind != ui.InputScroll || ev.DY == 0 || disabled {
 			return false
 		}
 		if ev.DY < 0 {