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/reorder_transition.go

@@ -20,7 +20,7 @@ type ReorderTransitionOptions struct {
 }
 
 // ReorderTransition slides keyed items to their new places.
-func ReorderTransition[K comparable](c *ui.Context, keys []K, opts ReorderTransitionOptions, item func(key K)) *ui.Element {
+func ReorderTransition[K comparable](c *ui.Context, keys []K, opts ReorderTransitionOptions, item func(key K)) ui.Element {
 	if opts.Duration < 0 || !(opts.Gap >= 0) || math.IsInf(float64(opts.Gap), 0) {
 		panic("mujica: ReorderTransition Duration and Gap must not be negative")
 	}
@@ -50,7 +50,7 @@ func ReorderTransition[K comparable](c *ui.Context, keys []K, opts ReorderTransi
 	st.keys = append(st.keys[:0], keys...)
 	col.Children(func() {
 		for _, k := range keys {
-			ui.Column(c).Key(k).AlignItems(ui.Stretch).Transition(move).Children(func() { item(k) })
+			ui.Column(c.Key(k)).AlignItems(ui.Stretch).Transition(move).Children(func() { item(k) })
 		}
 		for k, until := range st.leaving {
 			if seen[k] || !c.Now().Before(until) {
@@ -58,7 +58,7 @@ func ReorderTransition[K comparable](c *ui.Context, keys []K, opts ReorderTransi
 				continue
 			}
 			if c.Preferences().ReduceMotion {
-				ui.Column(c).Key(k).Absolute().Invisible().Transition(ui.ElementTransition{Duration: d, Position: true})
+				ui.Column(c.Key(k)).Absolute().Invisible().Transition(ui.ElementTransition{Duration: d, Position: true})
 				delete(st.leaving, k)
 			}
 		}