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>

display/text.go

@@ -19,7 +19,7 @@ type TextOptions struct {
 }
 
 // Text shows body text wrapping at its width.
-func Text(c *ui.Context, s string, opts TextOptions) *ui.Element {
+func Text(c *ui.Context, s string, opts TextOptions) ui.Element {
 	if opts.MaxLines < 0 {
 		panic("mujica: Text MaxLines must not be negative")
 	}
@@ -49,7 +49,7 @@ type EditableTextOptions struct {
 
 // EditableTextView is a shown EditableText and this frame's outcome.
 type EditableTextView struct {
-	Element *ui.Element
+	Element ui.Element
 	// Canceled reports Escape discarded an edit.
 	Canceled bool
 	changed  bool
@@ -118,7 +118,7 @@ func editableTextField(c *ui.Context, k theme.Tokens, value *string, opts Editab
 			return v
 		}
 	}
-	var in *ui.Element
+	var in ui.Element
 	box.Children(func() {
 		in = ui.TextInputBase(c, &st.draft).Grow(1).MinWidth(theme.MinHitSize*3).Padding(pad, pad*2).
 			Radius(theme.ControlRadius).Background(k.Surface).Border(theme.BorderWidth, k.Accent).TextColor(k.Text).