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/form_field_test.go

@@ -13,7 +13,7 @@ func TestFormFieldLabelFocusesControl(t *testing.T) {
 	name := ""
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
-		FormField(c, "Name", FormFieldOptions{Required: true, Description: "As on your seal."}, func() *ui.Element {
+		FormField(c, "Name", FormFieldOptions{Required: true, Description: "As on your seal."}, func() ui.Element {
 			return InputGroup(c, &name, InputGroupOptions{}).Input
 		})
 	}, 400, 200)
@@ -35,10 +35,10 @@ func TestFormFieldTargetsReturnedControl(t *testing.T) {
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
 		ui.Column(c).Gap(12).Children(func() {
-			FormField(c, "Flowers", FormFieldOptions{Error: "Too few."}, func() *ui.Element {
+			FormField(c, "Flowers", FormFieldOptions{Error: "Too few."}, func() ui.Element {
 				return TagsInput(c, &tags, TagsInputOptions{}).Input
 			})
-			FormField(c, "Site", FormFieldOptions{}, func() *ui.Element {
+			FormField(c, "Site", FormFieldOptions{}, func() ui.Element {
 				return InputGroup(c, &site, InputGroupOptions{Buttons: []InputGroupButton{{Label: "Back", Leading: true}}}).Input
 			})
 		})
@@ -64,7 +64,7 @@ func TestFormFieldErrorAndLayout(t *testing.T) {
 	horizontal := false
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
-		FormField(c, "Email", FormFieldOptions{Error: "Enter an email address.", Horizontal: horizontal}, func() *ui.Element {
+		FormField(c, "Email", FormFieldOptions{Error: "Enter an email address.", Horizontal: horizontal}, func() ui.Element {
 			return InputGroup(c, &v, InputGroupOptions{Prefix: "mailto:"}).Input
 		})
 	}, 500, 200)
@@ -93,7 +93,7 @@ func TestFormFieldNeedsLabel(t *testing.T) {
 	}()
 	ui.Render(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
-		FormField(c, "", FormFieldOptions{}, func() *ui.Element { return ui.Box(c) })
+		FormField(c, "", FormFieldOptions{}, func() ui.Element { return ui.Box(c) })
 	}, 100, 100, 1)
 }
 
@@ -101,10 +101,10 @@ func TestFormFieldSnapshot(t *testing.T) {
 	a, b := "Sakiko Togawa", "uika@"
 	snapshot(t, "form_field", 520, 260, func(c *ui.Context) {
 		ui.Column(c).Padding(20).Gap(16).Children(func() {
-			FormField(c, "Name", FormFieldOptions{Required: true, Description: "As written on the invitation."}, func() *ui.Element {
+			FormField(c, "Name", FormFieldOptions{Required: true, Description: "As written on the invitation."}, func() ui.Element {
 				return InputGroup(c, &a, InputGroupOptions{}).Input
 			})
-			FormField(c, "Email", FormFieldOptions{Horizontal: true, Error: "Enter a full email address."}, func() *ui.Element {
+			FormField(c, "Email", FormFieldOptions{Horizontal: true, Error: "Enter a full email address."}, func() ui.Element {
 				return InputGroup(c, &b, InputGroupOptions{Invalid: true}).Input
 			})
 		})