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_brush_test.go

@@ -14,14 +14,14 @@ type brushHarness struct {
 	value    [2]float64
 	changes  int
 	disabled bool
-	el       *ui.Element
+	bounds   ui.Rect
 }
 
 func (h *brushHarness) view(c *ui.Context) {
 	core.Use(c, core.Settings{Mode: core.Light})
 	ui.Column(c).Padding(10).Width(420).Children(func() {
 		r := ChartBrush(c, &h.value, ChartBrushOptions{Label: "Months", Min: 0, Max: 100, Step: 1, Disabled: h.disabled})
-		h.el = r.Element
+		h.bounds = r.Element.Bounds()
 		if r.Changed() {
 			h.changes++
 		}
@@ -57,7 +57,8 @@ func TestChartBrushKeyboard(t *testing.T) {
 func TestChartBrushDrag(t *testing.T) {
 	h := &brushHarness{value: [2]float64{20, 40}}
 	tt := ui.NewTester(h.view, 460, 200)
-	b := h.el.Bounds()
+	tt.Frame() // Bounds reads the previous frame
+	b := h.bounds
 	x := func(v float32) float32 { return b.X + 400*v/100 }
 	y := b.Y + 24
 	tt.Press(x(30), y) // inside the window
@@ -86,7 +87,8 @@ func TestChartBrushDrag(t *testing.T) {
 func TestChartBrushDisabledAndBad(t *testing.T) {
 	h := &brushHarness{value: [2]float64{20, 40}, disabled: true}
 	tt := ui.NewTester(h.view, 460, 200)
-	b := h.el.Bounds()
+	tt.Frame() // Bounds reads the previous frame
+	b := h.bounds
 	tt.Press(b.X+120, b.Y+24)
 	tt.Move(b.X+200, b.Y+24)
 	tt.Release(b.X+200, b.Y+24)
@@ -128,7 +130,8 @@ func TestChartBrushSnapshot(t *testing.T) {
 func TestChartBrushWindowDragPreservesSpanAtEdges(t *testing.T) {
 	h := &brushHarness{value: [2]float64{20, 60}}
 	tt := ui.NewTester(h.view, 460, 200)
-	b := h.el.Bounds()
+	tt.Frame() // Bounds reads the previous frame
+	b := h.bounds
 	tt.Press(b.X+160, b.Y+24)
 	tt.Move(b.X+380, b.Y+24)
 	tt.Release(b.X+380, b.Y+24)