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/masonry.go

@@ -46,7 +46,7 @@ type masonryLayout struct {
 }
 
 // Masonry lays n items in as many columns as fit, each into the shortest column, building only those in view.
-func Masonry(c *ui.Context, n int, opts MasonryOptions, item func(i int)) *ui.Element {
+func Masonry(c *ui.Context, n int, opts MasonryOptions, item func(i int)) ui.Element {
 	if opts.Key == nil || item == nil {
 		panic("mujica: Masonry needs Key and item")
 	}
@@ -98,7 +98,7 @@ func masonryBuild(c *ui.Context, s *masonryState, l *masonryLayout, colW, top, b
 			}
 			*seen = append(*seen, i)
 			key := l.keys[i]
-			box := ui.Column(c).Key(key).Absolute().Left(l.x[i]).Top(l.y[i]).Width(colW)
+			box := ui.Column(c.Key(key)).Absolute().Left(l.x[i]).Top(l.y[i]).Width(colW)
 			box.Children(func() { item(i) })
 			if h := box.Bounds().H; h > 0 && math.Abs(float64(h-l.h[i])) > 0.5 {
 				s.heights[key] = h