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/event_editor.go

@@ -24,7 +24,7 @@ type EventEditorOptions struct {
 
 // EventEditorResult is the editor and its save or cancel.
 type EventEditorResult struct {
-	Element  *ui.Element
+	Element  ui.Element
 	saved    bool
 	canceled bool
 }
@@ -61,7 +61,7 @@ func EventEditor(c *ui.Context, ev *CalendarEvent, opts EventEditorOptions) Even
 	dr := &s.draft
 	problems := eventEditorProblems(c, *dr, s.tried)
 	root.Children(func() {
-		input.FormField(c, core.Msg(c, "event_editor.title"), input.FormFieldOptions{Required: true, Error: problems["title"]}, func() *ui.Element {
+		input.FormField(c, core.Msg(c, "event_editor.title"), input.FormFieldOptions{Required: true, Error: problems["title"]}, func() ui.Element {
 			return input.TextInput(c, &dr.Title, input.TextInputOptions{Label: core.Msg(c, "event_editor.title"), Disabled: opts.Disabled, Error: problems["title"]}).Input
 		})
 		input.Checkbox(c, &dr.AllDay, core.Msg(c, "event_editor.all_day"), input.CheckboxOptions{Disabled: opts.Disabled})
@@ -70,7 +70,7 @@ func EventEditor(c *ui.Context, ev *CalendarEvent, opts EventEditorOptions) Even
 				key string
 				t   *time.Time
 			}{{"event_editor.starts", &dr.Start}, {"event_editor.ends", &dr.End}} {
-				input.FormField(c, core.Msg(c, f.key), input.FormFieldOptions{Error: problems[f.key]}, func() *ui.Element {
+				input.FormField(c, core.Msg(c, f.key), input.FormFieldOptions{Error: problems[f.key]}, func() ui.Element {
 					if dr.AllDay {
 						day := core.DateOf(f.t.In(loc))
 						r := DatePicker(c, &day, DatePickerOptions{Label: core.Msg(c, f.key), WeekStart: opts.WeekStart, Disabled: opts.Disabled})
@@ -84,10 +84,10 @@ func EventEditor(c *ui.Context, ev *CalendarEvent, opts EventEditorOptions) Even
 				})
 			}
 		})
-		input.FormField(c, core.Msg(c, "event_editor.where"), input.FormFieldOptions{}, func() *ui.Element {
+		input.FormField(c, core.Msg(c, "event_editor.where"), input.FormFieldOptions{}, func() ui.Element {
 			return input.TextInput(c, &dr.Location, input.TextInputOptions{Label: core.Msg(c, "event_editor.where"), Disabled: opts.Disabled}).Input
 		})
-		input.FormField(c, core.Msg(c, "event_editor.guests"), input.FormFieldOptions{Description: core.Msg(c, "event_editor.guests_help")}, func() *ui.Element {
+		input.FormField(c, core.Msg(c, "event_editor.guests"), input.FormFieldOptions{Description: core.Msg(c, "event_editor.guests_help")}, func() ui.Element {
 			names := make([]string, len(dr.Attendees))
 			for i, a := range dr.Attendees {
 				names[i] = a.Name
@@ -101,11 +101,11 @@ func EventEditor(c *ui.Context, ev *CalendarEvent, opts EventEditorOptions) Even
 		if len(dr.Attendees) > 0 {
 			AttendeeList(c, dr.Attendees, AttendeeListOptions{Disabled: opts.Disabled})
 		}
-		input.FormField(c, core.Msg(c, "event_editor.reminders"), input.FormFieldOptions{}, func() *ui.Element {
+		input.FormField(c, core.Msg(c, "event_editor.reminders"), input.FormFieldOptions{}, func() ui.Element {
 			return ReminderPicker(c, &dr.Reminders, ReminderPickerOptions{Disabled: opts.Disabled}).Element
 		})
 		var rec RecurrenceResult
-		input.FormField(c, core.Msg(c, "recurrence_editor.repeat"), input.FormFieldOptions{}, func() *ui.Element {
+		input.FormField(c, core.Msg(c, "recurrence_editor.repeat"), input.FormFieldOptions{}, func() ui.Element {
 			rec = RecurrenceEditor(c, &dr.Repeat, RecurrenceEditorOptions{Start: core.DateOf(dr.Start.In(loc)), WeekStart: opts.WeekStart, Disabled: opts.Disabled})
 			return rec.Element
 		})