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_legend.go

@@ -29,7 +29,7 @@ type ChartLegendOptions struct {
 
 // ChartLegendResult exposes the element and legend events.
 type ChartLegendResult struct {
-	Element          *ui.Element
+	Element          ui.Element
 	toggled          string
 	expanded, folded bool
 }
@@ -68,7 +68,7 @@ func ChartLegend(c *ui.Context, hidden *map[string]bool, items []ChartLegendItem
 			shown = items[:opts.MaxItems]
 		}
 		for _, it := range shown {
-			ui.Row(c).Key(it.Name).Children(func() {
+			ui.Row(c.Key(it.Name)).Children(func() {
 				if name, ok := chartLegendEntry(c, hidden, it, res.Element.IsDisabled()); ok {
 					res.toggled = name
 				}
@@ -100,7 +100,7 @@ func chartLegendEntry(c *ui.Context, hidden *map[string]bool, it ChartLegendItem
 	k := core.Tokens(c)
 	u := core.Density(c).Unit()
 	on := hidden == nil || !(*hidden)[it.Name]
-	var e *ui.Element
+	var e ui.Element
 	switched := false
 	if hidden != nil {
 		e = ui.CheckboxBase(c, &on).Disabled(disabled).Label(it.Name)