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/dots_loader.go

@@ -24,7 +24,7 @@ type LoaderOptions struct {
 type loaderDraw func(p *ui.Painter, r ui.Rect, t float64, col ui.Color, still bool)
 
 // loaderMark builds a loader that repaints only while painted.
-func loaderMark(c *ui.Context, opts LoaderOptions, key string, aspect float32, period int64, draw loaderDraw) *ui.Element {
+func loaderMark(c *ui.Context, opts LoaderOptions, key string, aspect float32, period int64, draw loaderDraw) ui.Element {
 	if !(opts.Size >= 0) || math.IsInf(float64(opts.Size), 1) {
 		panic("mujica: loader Size must be finite and not negative")
 	}
@@ -44,7 +44,7 @@ func loaderMark(c *ui.Context, opts LoaderOptions, key string, aspect float32, p
 	still := c.Preferences().ReduceMotion
 	row := ui.Row(c).Gap(core.Density(c).Unit() * 2).AlignItems(ui.Center).Shrink(0).Role(ui.RoleProgress).Label(label)
 	row.Children(func() {
-		ui.Box(c).Key(still).Size(size*aspect, size).Shrink(0).Role(ui.RoleNone).Draw(func(p *ui.Painter, r ui.Rect) {
+		ui.Box(c.Key(still)).Size(size*aspect, size).Shrink(0).Role(ui.RoleNone).Draw(func(p *ui.Painter, r ui.Rect) {
 			t := 0.0
 			if !still {
 				t = float64(p.Now().UnixMilli()%period) / float64(period)
@@ -64,7 +64,7 @@ func loaderMark(c *ui.Context, opts LoaderOptions, key string, aspect float32, p
 func loaderWave(t float64) float32 { return float32(0.5 - 0.5*math.Cos(2*math.Pi*t)) }
 
 // DotsLoader shows three dots lighting up in turn.
-func DotsLoader(c *ui.Context, opts LoaderOptions) *ui.Element {
+func DotsLoader(c *ui.Context, opts LoaderOptions) ui.Element {
 	return loaderMark(c, opts, "dots_loader", 2.5, 1200, func(p *ui.Painter, r ui.Rect, t float64, col ui.Color, still bool) {
 		d := r.H * 0.36
 		for i := range 3 {