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/transfer.go

@@ -175,7 +175,7 @@ func transferPanel(c *ui.Context, st *transferState, side int, title string, ite
 			}
 			for _, it := range shown {
 				on := st.checked[it.ID]
-				ui.Box(c).Key(it.ID).Children(func() {
+				ui.Box(c.Key(it.ID)).Children(func() {
 					cb := ui.CheckboxBase(c, &on).Gap(d.Unit()*2).MinHeight(max(d.ControlHeight()-d.Unit(), theme.MinHitSize)).Padding(0, d.Unit()*2).
 						Radius(theme.ControlRadius).FillWidth().Disabled(it.Disabled)
 					if cb.Changed() {
@@ -203,7 +203,7 @@ func transferPartial(c *ui.Context, checked, enabled int) string {
 }
 
 // transferMark draws a check box's square inside box, which takes the focus ring.
-func transferMark(c *ui.Context, box *ui.Element, on, mixed bool) {
+func transferMark(c *ui.Context, box ui.Element, on, mixed bool) {
 	k := core.Tokens(c)
 	internal.Ring(box, k.Focus, theme.ControlRadius)
 	box.Children(func() {