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/violin_plot_test.go

@@ -20,7 +20,7 @@ func TestViolinPlotReadout(t *testing.T) {
 		t.Fatalf("readout %q", got)
 	}
 	tt.Key(0, ui.KeyEscape)
-	b := h.res.Element.Bounds()
+	b := h.box(tt)
 	tt.Move(b.X+52+(b.W-64)/6, b.Y+120)
 	if h.res.Point != 0 {
 		t.Fatalf("hover %d", h.res.Point)
@@ -62,13 +62,14 @@ func TestViolinPlotLargeDataAndDisabled(t *testing.T) {
 	h := &chartHarness{build: func(c *ui.Context) ChartResult {
 		return ViolinPlot(c, groups, ViolinPlotOptions{Frame: ChartFrame{Label: "Large violin"}})
 	}}
+	var shown *[]int
+	h.built = func(r ChartResult) { shown = ui.Local(r.Element, "mujica.violin.visible", func() []int { return nil }) }
 	tt := ui.NewTester(h.view, 520, 420)
 	tabTo(t, tt, "Large violin")
 	tt.Key(0, ui.KeyRight)
 	if chartReadout(tt) != "Sample · median 49.5 · quartiles 24.75–74.25 · 100000 values" {
 		t.Fatalf("large readout %q", chartReadout(tt))
 	}
-	shown := ui.Local(h.res.Element, "mujica.violin.visible", func() []int { return nil })
 	if len(*shown) != 1 {
 		t.Fatalf("built %d violins", len(*shown))
 	}