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>

overlay/split_button.go

@@ -59,7 +59,7 @@ func SplitButton(c *ui.Context, label string, opts SplitButtonOptions, build fun
 			ui.Box(c).Width(theme.BorderWidth).Background(k.OnAccent.Alpha(0.45)).Role(ui.RoleNone)
 		}
 		more := ui.ButtonBase(c).Role(ui.RoleMenuButton).Label(menuName).MinHeight(d.ControlHeight()).Width(theme.MinHitSize+d.Unit()).
-			Radius(0, theme.ControlRadius, theme.ControlRadius, 0).Tooltip(menuName)
+			Radius(0, theme.ControlRadius, theme.ControlRadius, 0)
 		splitButtonLook(c, k, more, opts.Variant, *open)
 		if opts.Variant == input.Secondary {
 			more.BorderWidth(theme.BorderWidth, theme.BorderWidth, theme.BorderWidth, 0)
@@ -69,6 +69,10 @@ func SplitButton(c *ui.Context, label string, opts SplitButtonOptions, build fun
 			*open = true
 			slog.Debug("mujica: split_button open")
 		}
+		// A tip coming and going ahead of the open menu would re-key its items.
+		if !*open {
+			more.Tooltip(menuName)
+		}
 		PopupMenuOpen(c, group, open, build)
 	})
 	if clicked {
@@ -78,7 +82,7 @@ func SplitButton(c *ui.Context, label string, opts SplitButtonOptions, build fun
 }
 
 // splitButtonLook colors one half as a Button of variant v would be.
-func splitButtonLook(c *ui.Context, k theme.Tokens, b *ui.Element, v input.ButtonVariant, open bool) {
+func splitButtonLook(c *ui.Context, k theme.Tokens, b ui.Element, v input.ButtonVariant, open bool) {
 	internal.Ring(b, k.Focus, theme.ControlRadius)
 	bg, fg, border := input.ButtonColors(k, v, b.Hovered() && !b.IsDisabled(), b.Pressed() || open)
 	b.Background(bg).TextColor(fg).Transition(core.ColorFade)