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>

datetime/datetime_review2_test.go

@@ -167,21 +167,23 @@ func TestTimePickerPopupPickClearsSegmentError(t *testing.T) {
 func TestFormFocusesDatePickerInput(t *testing.T) {
 	var st input.FormState
 	day := core.Date{}
-	var field *ui.Element
+	focused := false
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
+		var field ui.Element
 		input.Form(c, &st, input.FormOptions{}, func(f *input.FormScope) {
-			input.FormItem(c, f, "debut", &day, input.FormItemOptions[core.Date]{Label: "Debut date", Required: true}, func() *ui.Element {
+			input.FormItem(c, f, "debut", &day, input.FormItemOptions[core.Date]{Label: "Debut date", Required: true}, func() ui.Element {
 				r := DatePicker(c, &day, DatePickerOptions{Today: calendarToday})
 				field = r.Input
 				return r.Input
 			})
 		})
+		focused = field.Valid() && field.Focused()
 	}, 600, 500)
 	if err := tt.Click("Submit"); err != nil {
 		t.Fatal(err)
 	}
-	if field == nil || !field.Focused() || !tt.Focused("Debut date") {
+	if !focused || !tt.Focused("Debut date") {
 		t.Fatal("a failed submit should focus the DatePicker's text box")
 	}
 	tt.Type("2026-10-12")