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>

project/status_select.go

@@ -41,7 +41,7 @@ func StatusSelect(c *ui.Context, value *string, statuses []WorkflowStatus, opts
 	k := core.Tokens(c)
 	old := *value
 	pick := old
-	var s *ui.SelectParts[string]
+	var s ui.SelectParts[string]
 	root := ui.Row(c).Shrink(0).Disabled(opts.Disabled)
 	epoch := ui.Local(root, "status-epoch", func() struct {
 		disabled   bool
@@ -57,7 +57,7 @@ func StatusSelect(c *ui.Context, value *string, statuses []WorkflowStatus, opts
 		epoch.generation++
 	}
 	root.Children(func() {
-		ui.Row(c).Key(epoch.generation).Children(func() { s = ui.SelectBase(c, &pick) })
+		ui.Row(c.Key(epoch.generation)).Children(func() { s = ui.SelectBase(c, &pick) })
 	})
 	tr := s.Trigger
 	label := opts.Label
@@ -70,7 +70,7 @@ func StatusSelect(c *ui.Context, value *string, statuses []WorkflowStatus, opts
 		ui.Text(c, cur.Name).SingleLine().Grow(1).Shrink(1).TextColor(k.Text)
 	})
 	if !tr.IsDisabled() {
-		s.Popup(func(panel *ui.Element) {
+		s.Popup(func(panel ui.Element) {
 			input.SelectPanel(c, panel)
 			_, wh := c.Size()
 			ui.Scroll(c).MaxHeight(min(320, wh/2)).Children(func() {