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_test.go

@@ -27,7 +27,7 @@ func (p *formProbe) run(c *ui.Context) {
 
 func (p *formProbe) view(c *ui.Context) {
 	res := Form(c, &p.st, FormOptions{Horizontal: p.horizontal}, func(f *FormScope) {
-		FormItem(c, f, "name", &p.name, FormItemOptions[string]{Label: "Name", Required: true, Initial: "Sakiko"}, func() *ui.Element {
+		FormItem(c, f, "name", &p.name, FormItemOptions[string]{Label: "Name", Required: true, Initial: "Sakiko"}, func() ui.Element {
 			return InputGroup(c, &p.name, InputGroupOptions{Invalid: p.st.Error("name") != ""}).Input
 		})
 		opts := FormItemOptions[string]{Label: "Email", Validate: func(v string) string {
@@ -42,10 +42,10 @@ func (p *formProbe) view(c *ui.Context) {
 				p.asyncValues = append(p.asyncValues, v)
 			}
 		}
-		FormItem(c, f, "email", &p.email, opts, func() *ui.Element {
+		FormItem(c, f, "email", &p.email, opts, func() ui.Element {
 			return InputGroup(c, &p.email, InputGroupOptions{Invalid: p.st.Error("email") != ""}).Input
 		})
-		FormItem(c, f, "tags", &p.tags, FormItemOptions[[]string]{Label: "Tags", Required: true, Initial: []string{"rose"}}, func() *ui.Element {
+		FormItem(c, f, "tags", &p.tags, FormItemOptions[[]string]{Label: "Tags", Required: true, Initial: []string{"rose"}}, func() ui.Element {
 			return TagsInput(c, &p.tags, TagsInputOptions{}).Input
 		})
 	})
@@ -224,7 +224,7 @@ func TestFormResetClearsControlState(t *testing.T) {
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
 		Form(c, &st, FormOptions{}, func(f *FormScope) {
-			FormItem(c, f, "tags", &tags, FormItemOptions[[]string]{Label: "Tags"}, func() *ui.Element {
+			FormItem(c, f, "tags", &tags, FormItemOptions[[]string]{Label: "Tags"}, func() ui.Element {
 				return TagsInput(c, &tags, TagsInputOptions{Validate: func(s string) string {
 					seen = s
 					return "Invalid tag"
@@ -291,10 +291,10 @@ func TestFormCrossFieldRuleAtSettle(t *testing.T) {
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
 		res := Form(c, &st, FormOptions{}, func(f *FormScope) {
-			FormItem(c, f, "user", &user, FormItemOptions[string]{Label: "User", Async: func(_ string, v uint64) { version = v }}, func() *ui.Element {
+			FormItem(c, f, "user", &user, FormItemOptions[string]{Label: "User", Async: func(_ string, v uint64) { version = v }}, func() ui.Element {
 				return InputGroup(c, &user, InputGroupOptions{}).Input
 			})
-			FormItem(c, f, "pw", &pw, FormItemOptions[string]{Label: "Password"}, func() *ui.Element {
+			FormItem(c, f, "pw", &pw, FormItemOptions[string]{Label: "Password"}, func() ui.Element {
 				return InputGroup(c, &pw, InputGroupOptions{}).Input
 			})
 			FormItem(c, f, "confirm", &confirm, FormItemOptions[string]{Label: "Confirm", Validate: func(v string) string {
@@ -302,7 +302,7 @@ func TestFormCrossFieldRuleAtSettle(t *testing.T) {
 					return "Passwords differ."
 				}
 				return ""
-			}}, func() *ui.Element {
+			}}, func() ui.Element {
 				return InputGroup(c, &confirm, InputGroupOptions{}).Input
 			})
 		})