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_frame.go

@@ -26,7 +26,7 @@ type ChartFrame struct {
 
 // ChartResult is a built chart and this frame's events.
 type ChartResult struct {
-	Element *ui.Element
+	Element ui.Element
 	// Point is the hovered or focused mark, -1 for none.
 	Point          int
 	changed, retry bool
@@ -69,7 +69,7 @@ type ChartSpec struct {
 	// Tooltip writes mark i's tooltip; nil splits Readout at " · ".
 	Tooltip func(i int) []ChartTooltipLine
 	// Input takes the plot's other input, as a ChartAnnotation's drags.
-	Input func(plot *ui.Element, r ui.Rect)
+	Input func(plot ui.Element, r ui.Rect)
 }
 
 // Chart frames a custom chart: legend, focus, keys, readout, tooltip and empty states.
@@ -120,10 +120,10 @@ func (ch *chart) Toggles(on bool) *map[string]bool {
 func (ch *chart) Hidden(name string) bool { return ch.st.hidden[name] }
 
 // Build lays out legend, plot and readout; returns the plot.
-func (ch *chart) Build(spec ChartSpec) (ChartResult, *ui.Element) {
+func (ch *chart) Build(spec ChartSpec) (ChartResult, ui.Element) {
 	c, k, st := ch.c, core.Tokens(ch.c), ch.st
 	u := core.Density(c).Unit()
-	var plot *ui.Element
+	var plot ui.Element
 	ch.Res.Element.Children(func() {
 		if len(spec.Legend) > 0 {
 			if name, ok := ChartLegend(c, spec.Hidden, spec.Legend, ChartLegendOptions{}).Toggled(); ok {
@@ -181,7 +181,7 @@ func (ch *chart) Build(spec ChartSpec) (ChartResult, *ui.Element) {
 }
 
 // input moves the keyboard mark, and takes clicks on marks.
-func (ch *chart) input(plot *ui.Element, spec ChartSpec) {
+func (ch *chart) input(plot ui.Element, spec ChartSpec) {
 	st := ch.st
 	if plot.IsDisabled() {
 		return