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>

chart/chart_empty_state_test.go

@@ -13,30 +13,33 @@ func TestChartEmptyStateKeepsSizeAndRetries(t *testing.T) {
 	status := data.DataReady
 	retries, disabled := 0, false
 	var heights []float32
-	var el *ui.Element
+	var box ui.Rect
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
 		ui.Column(c).Width(400).Disabled(disabled).Children(func() {
 			r := ChartEmptyState(c, status, ChartEmptyStateOptions{Height: 180, Empty: "No hymns this week", Error: "The choir did not answer"})
-			el = r.Element
+			box = r.Element.Bounds()
 			if r.Changed() {
 				retries++
 			}
 		})
 	}, 440, 300)
-	heights = append(heights, el.Bounds().H)
+	tt.Frame() // Bounds reads the previous frame
+	heights = append(heights, box.H)
 	if !tt.HasText("No hymns this week") {
 		t.Fatalf("empty: %q", tt.Texts())
 	}
 	status = data.DataLoading
 	tt.Frame()
-	heights = append(heights, el.Bounds().H)
+	tt.Frame() // Bounds reads the previous frame
+	heights = append(heights, box.H)
 	if !tt.HasText("Loading") {
 		t.Fatalf("loading: %q", tt.Texts())
 	}
 	status = data.DataFailed
 	tt.Frame()
-	heights = append(heights, el.Bounds().H)
+	tt.Frame() // Bounds reads the previous frame
+	heights = append(heights, box.H)
 	if err := tt.Click("Retry"); err != nil || retries != 1 || !tt.HasText("The choir did not answer") {
 		t.Fatalf("retry: %v %d", err, retries)
 	}