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>

input/radio_group.go

@@ -71,8 +71,8 @@ func RadioGroup[T comparable](c *ui.Context, value *T, items []RadioItem[T], opt
 // radioGroupItem builds one radio and reports whether it was chosen.
 func radioGroupItem[T comparable](c *ui.Context, k theme.Tokens, d theme.Density, value *T, it RadioItem[T], key int) bool {
 	// Keyed by value so focus follows the item, not its position.
-	var row *ui.Element
-	item := ui.Box(c).Key(key).Disabled(it.Disabled)
+	var row ui.Element
+	item := ui.Box(c.Key(key)).Disabled(it.Disabled)
 	item.Children(func() {
 		row = ui.RadioBase(c, value, it.Value).Gap(d.Unit() * 2).MinHeight(d.ControlHeight()).FocusRing(false)
 	})