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>

devtools/helpers_test.go

@@ -16,15 +16,14 @@ func snapshot(t *testing.T, name string, w, h int, view func(c *ui.Context)) {
 	snaptest.Run(t, name, w, h, snapshotTheme, view, nil)
 }
 
-// overlayUntil draws frames until cond holds, for at most 5 s of wall time.
+// overlayUntil advances the Tester clock until cond holds, for at most 5 s.
 func overlayUntil(tt *ui.Tester, cond func() bool) bool {
-	end := time.Now().Add(5 * time.Second)
+	end := tt.Now().Add(5 * time.Second)
 	for !cond() {
-		if time.Now().After(end) {
+		if tt.Now().After(end) {
 			return false
 		}
-		time.Sleep(2 * time.Millisecond)
-		tt.Frame()
+		tt.Advance(2 * time.Millisecond)
 	}
 	return true
 }