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>

devtools/refresh_interval_selector_test.go

@@ -31,8 +31,7 @@ func TestRefreshIntervalSelectorManualAndAuto(t *testing.T) {
 	if refreshes != 1 {
 		t.Fatalf("manual refreshes %d", refreshes)
 	}
-	time.Sleep(1100 * time.Millisecond)
-	tt.Frame()
+	tt.Advance(1100 * time.Millisecond)
 	if refreshes != 1 {
 		t.Fatal("refreshed while off")
 	}
@@ -41,11 +40,11 @@ func TestRefreshIntervalSelectorManualAndAuto(t *testing.T) {
 	if interval != time.Second || changes != 1 {
 		t.Fatalf("interval %v changes %d", interval, changes)
 	}
-	start := time.Now()
+	start := tt.Now()
 	if !overlayUntil(tt, func() bool { return refreshes == 2 }) {
 		t.Fatal("no automatic refresh")
 	}
-	if time.Since(start) < 900*time.Millisecond {
+	if tt.Now().Sub(start) < 900*time.Millisecond {
 		t.Fatal("refreshed before the interval")
 	}
 }
@@ -56,15 +55,13 @@ func TestRefreshIntervalSelectorRefreshingAndDisabled(t *testing.T) {
 	refreshes, changes := 0, 0
 	tt := refreshTester(&interval, &opts, &refreshes, &changes)
 	_ = tt.Click("Refresh")
-	time.Sleep(1100 * time.Millisecond)
-	tt.Frame()
+	tt.Advance(1100 * time.Millisecond)
 	if refreshes != 0 || !tt.HasText("Refresh") {
 		t.Fatalf("refreshing: %d", refreshes)
 	}
 	opts = RefreshIntervalSelectorOptions{Intervals: []time.Duration{time.Second}, Disabled: true}
 	tt.Frame()
-	time.Sleep(1100 * time.Millisecond)
-	tt.Frame()
+	tt.Advance(1100 * time.Millisecond)
 	_ = tt.Click("Refresh")
 	if refreshes != 0 {
 		t.Fatalf("disabled refreshed %d", refreshes)