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/combobox.go

@@ -54,16 +54,10 @@ func Combobox(c *ui.Context, value *string, options []string, opts ComboboxOptio
 	// MyGo keeps the caret on replaced text (ui/editor.go:117): a choice builds a fresh input.
 	gen := ui.Local(f, comboboxKey{"input"}, func() int { return 0 })
 	refocus := ui.Local(f, comboboxKey{"refocus"}, func() bool { return false })
-	var p *ui.ComboboxParts
+	var p ui.ComboboxParts
 	f.Children(func() {
-		// A focused ComboboxBase takes typing under a disabled wrapper, so it edits a copy.
-		target := query
-		if opts.Disabled {
-			cp := *query
-			target = &cp
-		}
-		ui.Row(c).Key(*gen).Grow(1).Children(func() {
-			p = ui.ComboboxBase(c, target)
+		ui.Row(c.Key(*gen)).Grow(1).Children(func() {
+			p = ui.ComboboxBase(c, query)
 		})
 		if *refocus {
 			p.Input.Focus()
@@ -113,7 +107,7 @@ func Combobox(c *ui.Context, value *string, options []string, opts ComboboxOptio
 	if !p.Open() {
 		*lastHi = -1
 	}
-	p.Popup(func(panel *ui.Element) {
+	p.Popup(func(panel ui.Element) {
 		SelectPanel(c, panel)
 		ui.Scroll(c).MaxHeight(240).Children(func() {
 			if custom != "" {