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/line_chart_test.go

@@ -14,15 +14,27 @@ import (
 
 type chartHarness struct {
 	res      ChartResult
+	bounds   ui.Rect
 	changes  int
 	disabled bool
 	build    func(c *ui.Context) ChartResult
+	built    func(r ChartResult) // reads the live element during the build
+}
+
+// box is the plot's committed bounds: Bounds reads the previous frame, so one more frame settles it.
+func (h *chartHarness) box(tt *ui.Tester) ui.Rect {
+	tt.Frame()
+	return h.bounds
 }
 
 func (h *chartHarness) view(c *ui.Context) {
 	core.Use(c, core.Settings{Mode: core.Light})
 	ui.Column(c).Padding(16).Width(480).Disabled(h.disabled).Children(func() {
 		h.res = h.build(c)
+		h.bounds = h.res.Element.Bounds()
+		if h.built != nil {
+			h.built(h.res)
+		}
 		if h.res.Changed() {
 			h.changes++
 		}
@@ -84,7 +96,7 @@ func TestLineChartHoverAndToggle(t *testing.T) {
 		return r
 	}}
 	tt := ui.NewTester(h.view, 520, 420)
-	b := h.res.Element.Bounds()
+	b := h.box(tt)
 	// The plot spans 52..W-12; x 30 is three quarters across.
 	px := b.X + 52 + (b.W-64)*0.75
 	tt.Move(px, b.Y+b.H/2)
@@ -118,11 +130,11 @@ func TestLineChartTimeAxisAndStates(t *testing.T) {
 	if got := chartReadout(tt); !strings.HasPrefix(got, "2026-10-05 10:00:00 · ") {
 		t.Fatalf("time readout %q", got)
 	}
-	heights = append(heights, h.res.Element.Bounds().H)
+	heights = append(heights, h.box(tt).H)
 	for _, st := range []data.DataStatus{data.DataLoading, data.DataFailed} {
 		status = st
 		tt.Frame()
-		heights = append(heights, h.res.Element.Bounds().H)
+		heights = append(heights, h.box(tt).H)
 	}
 	if !tt.HasText("Retry") || heights[0] != heights[1] || heights[1] != heights[2] {
 		t.Fatalf("states changed size: %v", heights)