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>

media/image_annotator.go

@@ -185,7 +185,7 @@ func annotatorPreview(s *annotatorState) []Annotation {
 }
 
 // annotatorPointer draws shapes, selects by click, drags the selection.
-func annotatorPointer(stage *ui.Element, s *annotatorState, notes *[]Annotation, r ui.Rect, size image.Point, scale, ox, oy float32) {
+func annotatorPointer(stage ui.Element, s *annotatorState, notes *[]Annotation, r ui.Rect, size image.Point, scale, ox, oy float32) {
 	_, _, _ = stage.Dragged()
 	px, py, _ := stage.PointerPosition()
 	pt := image.Pt(int(math.Round(float64((px+r.X-ox)/scale))), int(math.Round(float64((py+r.Y-oy)/scale))))
@@ -218,7 +218,7 @@ func annotatorPointer(stage *ui.Element, s *annotatorState, notes *[]Annotation,
 }
 
 // annotatorKeys: [ ] select, arrows move, Delete removes, Escape clears.
-func annotatorKeys(stage *ui.Element, s *annotatorState, notes *[]Annotation, size image.Point) {
+func annotatorKeys(stage ui.Element, s *annotatorState, notes *[]Annotation, size image.Point) {
 	n := len(*notes)
 	step := max(1, size.X/100)
 	move := func(dx, dy int) {