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>

data/list.go

@@ -67,7 +67,7 @@ func (s *ListState[K]) ScrollTo(i int) {
 
 // ListView is a built List and what happened to it in this frame.
 type ListView struct {
-	Element                   *ui.Element
+	Element                   ui.Element
 	changed, submitted, retry bool
 }
 
@@ -100,7 +100,7 @@ func List[K comparable](c *ui.Context, s *ListState[K], n int, opts ListOptions[
 			v.retry = DataPlaceholder(c, opts.Status, opts.Error, opts.Empty, core.Msg(c, "list.empty"))
 			return
 		}
-		var e *ui.Element
+		var e ui.Element
 		if opts.Grid {
 			e = listGrid(c, s, n, opts, item)
 		} else {
@@ -136,7 +136,7 @@ func listLocate[K comparable](s *ListState[K], n int, key func(int) K) {
 	slog.Debug("mujica: list selection moved", "row", s.cursor)
 }
 
-func listRows[K comparable](c *ui.Context, s *ListState[K], n int, opts ListOptions[K], item func(i int)) *ui.Element {
+func listRows[K comparable](c *ui.Context, s *ListState[K], n int, opts ListOptions[K], item func(i int)) ui.Element {
 	d := core.Density(c)
 	s.Rows.Key = func(i int) any { return opts.Key(i) }
 	s.Rows.Label = opts.Label
@@ -159,7 +159,7 @@ func listRows[K comparable](c *ui.Context, s *ListState[K], n int, opts ListOpti
 	}).Padding(d.Unit())
 }
 
-func listGrid[K comparable](c *ui.Context, s *ListState[K], n int, opts ListOptions[K], item func(i int)) *ui.Element {
+func listGrid[K comparable](c *ui.Context, s *ListState[K], n int, opts ListOptions[K], item func(i int)) ui.Element {
 	d := core.Density(c)
 	w, h := opts.ItemWidth, opts.ItemHeight
 	if w <= 0 {