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/slider.go

@@ -78,7 +78,7 @@ type sliderKey struct {
 type sliderKeyID struct{}
 
 // SliderControl builds a SliderBase on a copy of v and returns where the pointer or a key moves v.
-func SliderControl(c *ui.Context, v float64, g SliderGeom, from, to float64) (s *ui.Element, next float64, dragged bool) {
+func SliderControl(c *ui.Context, v float64, g SliderGeom, from, to float64) (s ui.Element, next float64, dragged bool) {
 	raw := v
 	s = ui.SliderBase(c, &raw, g.lo, g.hi)
 	ks := *ui.Local(s, sliderKeyID{}, func() *sliderKey { return &sliderKey{} })
@@ -175,7 +175,7 @@ func Slider(c *ui.Context, value *float64, opts SliderOptions) core.ChoiceResult
 	k := core.Tokens(c)
 	old := g.snap(*value)
 	*value = old
-	var s *ui.Element
+	var s ui.Element
 	var next float64
 	var held bool
 	// Disabled on a wrapper blocks the base before it takes input.