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/multi_select.go

@@ -55,7 +55,7 @@ func MultiSelect[T comparable](c *ui.Context, values *[]T, options []SelectOptio
 	SelectField(c, tr, *open, func() {
 		MultiSelectTags(c, values, options, opts)
 	})
-	ui.PopoverBase(c, tr, open, func(panel *ui.Element) {
+	ui.PopoverBase(c, tr, open, func(panel ui.Element) {
 		SelectPanel(c, panel)
 		panel.MinWidth(tr.Bounds().W).MaxWidth(max(tr.Bounds().W, 320))
 		multiSelectPopup(c, k, values, options, query, opts)
@@ -84,7 +84,7 @@ func MultiSelectTags[T comparable](c *ui.Context, values *[]T, options []SelectO
 				break
 			}
 			label, _ := SelectLabel(options, v)
-			ui.Row(c).Key(v).Shrink(0).Children(func() {
+			ui.Row(c.Key(v)).Shrink(0).Children(func() {
 				if multiSelectTag(c, label, opts.Disabled) {
 					*values = slices.Delete(slices.Clone(*values), i, i+1)
 				}
@@ -160,7 +160,7 @@ func multiSelectPopup[T comparable](c *ui.Context, k theme.Tokens, values *[]T,
 					if on {
 						st = internal.Checked
 					}
-					row := ui.Row(c).Key(o.Value)
+					row := ui.Row(c.Key(o.Value))
 					row.Children(func() {
 						cb := MultiSelectCheck(c, k, &on, st, o.Label, o.Disabled || (full && !on))
 						if cb.Changed() {
@@ -178,9 +178,9 @@ func multiSelectPopup[T comparable](c *ui.Context, k theme.Tokens, values *[]T,
 }
 
 // MultiSelectCheck is a CheckboxBase row drawing its own box.
-func MultiSelectCheck(c *ui.Context, k theme.Tokens, on *bool, st internal.CheckState, label string, disabled bool) *ui.Element {
+func MultiSelectCheck(c *ui.Context, k theme.Tokens, on *bool, st internal.CheckState, label string, disabled bool) ui.Element {
 	d := core.Density(c)
-	var cb *ui.Element
+	var cb ui.Element
 	// A disabled wrapper blocks the base, which toggles as it is built.
 	ui.Row(c).Grow(1).Disabled(disabled).Children(func() {
 		cb = ui.CheckboxBase(c, on).MinHeight(d.ControlHeight()).Padding(0, d.Unit()*2).
@@ -227,17 +227,17 @@ func MultiSelectAll[T comparable](values []T, shown []SelectOption[T], on bool,
 }
 
 // SelectSearch is the search field at the top of a popup; it takes the focus as the popup opens.
-func SelectSearch(c *ui.Context, query *string, placeholder string) *ui.Element {
+func SelectSearch(c *ui.Context, query *string, placeholder string) ui.Element {
 	k := core.Tokens(c)
 	d := core.Density(c)
 	f := ui.Row(c).Gap(d.Unit()*2).Padding(0, d.Unit()*2).MinHeight(d.ControlHeight()-4).AlignItems(ui.Center).
 		Radius(theme.ControlRadius).Background(k.Background).Border(theme.BorderWidth, k.ControlBorder)
-	var in *ui.Element
+	var in ui.Element
 	f.Children(func() {
 		ui.Box(c).Size(14, 14).Shrink(0).Draw(func(p *ui.Painter, r ui.Rect) {
 			internal.Mark(p, r, "search", k.TextMuted)
 		})
-		in = ui.TextInputBase(c, query).Grow(1).Placeholder(placeholder).Label(placeholder).AutoFocus()
+		in = core.AutoFocus(c, ui.TextInputBase(c, query).Grow(1).Placeholder(placeholder).Label(placeholder))
 	})
 	if in.Focused() {
 		f.Border(theme.BorderWidth, k.AccentText)