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/toggle.go

@@ -20,11 +20,11 @@ type ToggleOptions struct {
 }
 
 // Toggle is a button that stays pressed while *on. Click or Space flips it; Changed reports it.
-func Toggle(c *ui.Context, on *bool, label string, opts ToggleOptions) *ui.Element {
+func Toggle(c *ui.Context, on *bool, label string, opts ToggleOptions) ui.Element {
 	if label == "" && opts.Label == "" {
 		panic("mujica: Toggle needs a label or Label")
 	}
-	var b *ui.Element
+	var b ui.Element
 	ui.Row(c).Shrink(0).Disabled(opts.Disabled).Children(func() {
 		b = ui.ToggleBase(c, on)
 	})
@@ -39,7 +39,7 @@ func Toggle(c *ui.Context, on *bool, label string, opts ToggleOptions) *ui.Eleme
 }
 
 // toggleLook styles a pressed-or-not button and builds its content.
-func toggleLook(c *ui.Context, b *ui.Element, on bool, label string, icon *ui.SVG) {
+func toggleLook(c *ui.Context, b ui.Element, on bool, label string, icon *ui.SVG) {
 	k := core.Tokens(c)
 	d := core.Density(c)
 	b.MinHeight(d.ControlHeight()).MinWidth(d.ControlHeight()).Padding(0, d.Unit()*3).Gap(d.Unit() * 2).
@@ -69,7 +69,7 @@ func toggleLook(c *ui.Context, b *ui.Element, on bool, label string, icon *ui.SV
 }
 
 // toggleMark draws a bar under the content: the pressed state beside the color.
-func toggleMark(b *ui.Element, k theme.Tokens) {
+func toggleMark(b ui.Element, k theme.Tokens) {
 	b.Draw(func(p *ui.Painter, r ui.Rect) {
 		p.Fill(ui.Rect{X: r.X + r.W*0.25, Y: r.Y + r.H - 4, W: r.W * 0.5, H: 2}, k.AccentText, 1)
 	})