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>

code/completion_menu.go

@@ -34,7 +34,7 @@ type CompletionMenuOptions struct {
 
 // CompletionMenuResult carries the accepted candidate.
 type CompletionMenuResult struct {
-	Element  *ui.Element
+	Element  ui.Element
 	Accepted *Completion
 }
 
@@ -42,8 +42,8 @@ type CompletionMenuResult struct {
 const completionRows = 8
 
 // CompletionMenu shows candidates under the editor anchor.
-func CompletionMenu(c *ui.Context, anchor *ui.Element, open *bool, items []Completion, s *CompletionMenuState, opts CompletionMenuOptions) CompletionMenuResult {
-	if anchor == nil || open == nil || s == nil {
+func CompletionMenu(c *ui.Context, anchor ui.Element, open *bool, items []Completion, s *CompletionMenuState, opts CompletionMenuOptions) CompletionMenuResult {
+	if !anchor.Valid() || open == nil || s == nil {
 		panic("mujica: CompletionMenu needs an anchor, open and a state")
 	}
 	for _, it := range items {
@@ -70,8 +70,9 @@ func CompletionMenu(c *ui.Context, anchor *ui.Element, open *bool, items []Compl
 		s.rows.ScrollIntoView(s.Selected)
 	}
 	// Take the menu keys before the editor does.
+	disabled := anchor.IsDisabled()
 	anchor.HandleInput(func(ev ui.InputEvent) bool {
-		if ev.Kind != ui.InputKeyDown || ev.Mods != 0 || !*open || anchor.IsDisabled() {
+		if ev.Kind != ui.InputKeyDown || ev.Mods != 0 || !*open || disabled {
 			return false
 		}
 		switch ev.Key {
@@ -100,7 +101,7 @@ func CompletionMenu(c *ui.Context, anchor *ui.Element, open *bool, items []Compl
 	d := core.Density(c)
 	b := anchor.Bounds()
 	core.OverlayLayer(c, open)
-	res.Element = ui.PopoverBase(c, anchor, open, func(panel *ui.Element) {
+	res.Element = ui.PopoverBase(c, anchor, open, func(panel ui.Element) {
 		core.OverlayStyle(c, panel, overlay.OverlayAnchored(c, panel, b))
 		panel.Margin(4, 0, 0, 0).Padding(d.Unit()).Width(overlay.PopoverWide(c, 360)).MaxHeight(overlay.PopoverRoom(c, b)).
 			Label(core.Msg(c, "completion_menu.label")).Role(ui.RoleList)