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/parallel_coordinates_test.go

@@ -29,7 +29,7 @@ func TestParallelCoordinatesReadoutAndBrush(t *testing.T) {
 	if got := chartReadout(tt); got != "Bede · Age 65 · Hymns 1 · Vigils no value" {
 		t.Fatalf("readout %q", got)
 	}
-	b := h.res.Element.Bounds()
+	b := h.box(tt)
 	plot := ui.Rect{X: b.X + 36, Y: b.Y + 24, W: b.W - 72, H: 240 - 32}
 	// Age runs 20..80 on the first axis: brush 30..55.
 	y := func(v float32) float32 { return plot.Y + plot.H*(1-(v-20)/60) }
@@ -58,7 +58,7 @@ func TestParallelCoordinatesHoverAndBad(t *testing.T) {
 		return ParallelCoordinates(c, &filters, pcAxes, pcRows, ParallelCoordinatesOptions{Frame: ChartFrame{Label: "Monks"}})
 	}}
 	tt := ui.NewTester(h.view, 520, 420)
-	b := h.res.Element.Bounds()
+	b := h.box(tt)
 	plot := ui.Rect{X: b.X + 36, Y: b.Y + 24, W: b.W - 72, H: 240 - 32}
 	// Row 2 crosses this point between the first axes.
 	tt.Move(plot.X+plot.W/8, plot.Y+plot.H*(1-0.53125))
@@ -121,8 +121,9 @@ func TestParallelCoordinatesLargeDataBounded(t *testing.T) {
 	h := &chartHarness{build: func(c *ui.Context) ChartResult {
 		return ParallelCoordinates(c, &filters, pcAxes, rows, ParallelCoordinatesOptions{Frame: ChartFrame{Label: "Large parallel"}})
 	}}
+	var shown *[]int
+	h.built = func(r ChartResult) { shown = ui.Local(r.Element, "mujica.pc.visible", func() []int { return nil }) }
 	tt := ui.NewTester(h.view, 520, 420)
-	shown := ui.Local(h.res.Element, "mujica.pc.visible", func() []int { return nil })
 	if len(*shown) == 0 || len(*shown) > 480 {
 		t.Fatalf("built %d rows", len(*shown))
 	}