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>

feedback/loaders_test.go

@@ -39,11 +39,11 @@ func motionCount(tt *ui.Tester, s string) int {
 	return n
 }
 
-var loaderCases = map[string]func(c *ui.Context, o LoaderOptions) *ui.Element{
+var loaderCases = map[string]func(c *ui.Context, o LoaderOptions) ui.Element{
 	"dots": DotsLoader, "bars": BarsLoader, "pulse": PulseLoader, "orbit": OrbitLoader, "wave": WaveLoader,
 }
 
-func testLoaderBehavior(t *testing.T, loader func(*ui.Context, LoaderOptions) *ui.Element) {
+func testLoaderBehavior(t *testing.T, loader func(*ui.Context, LoaderOptions) ui.Element) {
 	show := true
 	tt, n := motionFrames(func(c *ui.Context) {
 		if show {
@@ -57,8 +57,7 @@ func testLoaderBehavior(t *testing.T, loader func(*ui.Context, LoaderOptions) *u
 		t.Fatal("label shown without ShowLabel")
 	}
 	a := encode(t, tt.Image())
-	time.Sleep(120 * time.Millisecond)
-	tt.Frame()
+	tt.Advance(120 * time.Millisecond)
 	if bytes.Equal(a, encode(t, tt.Image())) || !motionBusy(tt, n) {
 		t.Fatal("loader does not move")
 	}
@@ -67,8 +66,7 @@ func testLoaderBehavior(t *testing.T, loader func(*ui.Context, LoaderOptions) *u
 		t.Fatalf("reduced motion: still repainting or no static label: %q", tt.Texts())
 	}
 	a = encode(t, tt.Image())
-	time.Sleep(120 * time.Millisecond)
-	tt.Frame()
+	tt.Advance(120 * time.Millisecond)
 	if !bytes.Equal(a, encode(t, tt.Image())) {
 		t.Fatal("loader moved under reduced motion")
 	}
@@ -96,7 +94,7 @@ func TestLoadersInvalidSize(t *testing.T) {
 	}
 }
 
-func loaderSnapshot(t *testing.T, name string, loader func(c *ui.Context, o LoaderOptions) *ui.Element) {
+func loaderSnapshot(t *testing.T, name string, loader func(c *ui.Context, o LoaderOptions) ui.Element) {
 	snapshot(t, name, 480, 90, func(c *ui.Context) {
 		ui.Row(c).Padding(20).Gap(24).AlignItems(ui.Center).Children(func() {
 			loader(c, LoaderOptions{})