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>

core/base_disabled_test.go

@@ -8,11 +8,11 @@ import (
 
 // TestBasesBlockedByDisabledWrapper flips a focused MyGo base's wrapper to Disabled in the frame Space arrives.
 func TestBasesBlockedByDisabledWrapper(t *testing.T) {
-	for name, base := range map[string]func(c *ui.Context, on *bool) *ui.Element{
-		"CheckboxBase": func(c *ui.Context, on *bool) *ui.Element { return ui.CheckboxBase(c, on) },
-		"RadioBase":    func(c *ui.Context, on *bool) *ui.Element { return ui.RadioBase(c, on, true) },
-		"SwitchBase":   func(c *ui.Context, on *bool) *ui.Element { return ui.SwitchBase(c, on) },
-		"ToggleBase":   func(c *ui.Context, on *bool) *ui.Element { return ui.ToggleBase(c, on) },
+	for name, base := range map[string]func(c *ui.Context, on *bool) ui.Element{
+		"CheckboxBase": func(c *ui.Context, on *bool) ui.Element { return ui.CheckboxBase(c, on) },
+		"RadioBase":    func(c *ui.Context, on *bool) ui.Element { return ui.RadioBase(c, on, true) },
+		"SwitchBase":   func(c *ui.Context, on *bool) ui.Element { return ui.SwitchBase(c, on) },
+		"ToggleBase":   func(c *ui.Context, on *bool) ui.Element { return ui.ToggleBase(c, on) },
 	} {
 		for _, disable := range []bool{false, true} {
 			on, off := false, false
@@ -35,8 +35,8 @@ func TestBasesBlockedByDisabledWrapper(t *testing.T) {
 	}
 }
 
-// TestComboboxBaseIgnoresDisabledWrapper shows why Combobox hands its base a copy: a focused one still takes typing.
-func TestComboboxBaseIgnoresDisabledWrapper(t *testing.T) {
+// TestComboboxBaseBlockedByDisabledWrapper guards Combobox, which binds its base directly: a focused one takes no typing.
+func TestComboboxBaseBlockedByDisabledWrapper(t *testing.T) {
 	q, off := "", false
 	tt := ui.NewTester(func(c *ui.Context) {
 		ui.Row(c).Disabled(off).Children(func() {
@@ -48,9 +48,10 @@ func TestComboboxBaseIgnoresDisabledWrapper(t *testing.T) {
 		t.Fatal("not focused")
 	}
 	off = true
+	tt.Frame()
 	tt.Type("a")
 	tt.Frame()
-	if q != "a" {
-		t.Fatalf("typing left %q; the wrapper now blocks it and Combobox can drop its copy", q)
+	if q != "" {
+		t.Fatalf("typing under a disabled wrapper set %q; Combobox must edit a copy again", q)
 	}
 }