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>

input/select.go

@@ -53,7 +53,7 @@ func Select[T comparable](c *ui.Context, value *T, options []SelectOption[T], op
 	old := *value
 	// SelectBase chooses into a copy as it is built; a choice counts once the option is known enabled.
 	pick := old
-	var s *ui.SelectParts[T]
+	var s ui.SelectParts[T]
 	site := ui.Row(c).Shrink(0).Disabled(opts.Disabled)
 	life := ui.Local(site, selectKey{"lifetime"}, func() selectLifetime { return selectLifetime{} })
 	if disabled := site.IsDisabled(); disabled != life.disabled {
@@ -61,7 +61,7 @@ func Select[T comparable](c *ui.Context, value *T, options []SelectOption[T], op
 		life.generation++
 	}
 	site.Children(func() {
-		ui.Row(c).Key(life.generation).Children(func() { s = ui.SelectBase(c, &pick) })
+		ui.Row(c.Key(life.generation)).Children(func() { s = ui.SelectBase(c, &pick) })
 	})
 	tr := s.Trigger
 	commit := func() {
@@ -75,18 +75,19 @@ func Select[T comparable](c *ui.Context, value *T, options []SelectOption[T], op
 		tr.Label(opts.Label)
 	}
 	ta := ui.Local(tr, selectKey{"typeahead"}, func() selectTypeahead { return selectTypeahead{} })
+	disabled := tr.IsDisabled()
 	tr.TextCaret(ui.Rect{W: 1, H: core.Density(c).ControlHeight()}).HandleInput(func(ev ui.InputEvent) bool {
-		if tr.IsDisabled() {
+		if disabled {
 			return false
 		}
 		if ev.Kind != ui.InputText || strings.TrimSpace(ev.Text) == "" {
 			return false
 		}
-		if time.Since(ta.at) > time.Second {
+		if ev.Time.Sub(ta.at) > time.Second {
 			ta.prefix = ""
 		}
 		ta.prefix += strings.ToLower(ev.Text)
-		ta.pending, ta.at = true, time.Now()
+		ta.pending, ta.at = true, ev.Time
 		return true
 	})
 	if ta.pending {
@@ -117,7 +118,7 @@ func Select[T comparable](c *ui.Context, value *T, options []SelectOption[T], op
 	if tr.IsDisabled() {
 		return core.NewChoiceResult(tr, false)
 	}
-	s.Popup(func(panel *ui.Element) {
+	s.Popup(func(panel ui.Element) {
 		SelectPanel(c, panel)
 		ui.Scroll(c).MaxHeight(280).Children(func() {
 			if len(options) == 0 {
@@ -196,7 +197,7 @@ func selectOr(s, fallback string) string {
 }
 
 // SelectField styles tr as a field that opens a popup, with content and a chevron.
-func SelectField(c *ui.Context, tr *ui.Element, open bool, content func()) {
+func SelectField(c *ui.Context, tr ui.Element, open bool, content func()) {
 	k := core.Tokens(c)
 	d := core.Density(c)
 	tr.MinHeight(d.ControlHeight()).MinWidth(180).Padding(0, d.Unit()*2, 0, d.Unit()*3).Gap(d.Unit() * 2).
@@ -254,13 +255,13 @@ func SelectCheck(c *ui.Context, on bool) {
 }
 
 // SelectPanel styles a popup panel.
-func SelectPanel(c *ui.Context, panel *ui.Element) {
+func SelectPanel(c *ui.Context, panel ui.Element) {
 	core.OverlayStyle(c, panel, core.OverlayEnter(c, panel))
 	panel.Margin(4, 0, 0, 0).Padding(4)
 }
 
 // SelectRow styles an option row: chosen has the selection fill and a check, highlighted a side bar.
-func SelectRow(c *ui.Context, row *ui.Element, highlighted, chosen bool, label string) {
+func SelectRow(c *ui.Context, row ui.Element, highlighted, chosen bool, label string) {
 	k := core.Tokens(c)
 	d := core.Density(c)
 	row.MinHeight(d.ControlHeight()).Padding(0, d.Unit()*3, 0, d.Unit()*2).Gap(d.Unit() * 2).