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>

data/image_viewer.go

@@ -47,7 +47,7 @@ func ImageViewer(c *ui.Context, open *bool, index *int, images []ViewerImage) co
 	k := core.Tokens(c)
 	u := core.Density(c).Unit()
 	w, h := c.Size()
-	v.Element = ui.DialogBase(c, open, func(back, panel *ui.Element) {
+	v.Element = ui.DialogBase(c, open, func(back, panel ui.Element) {
 		back.Background(ui.RGBA(0, 0, 0, 0.55))
 		panel.Background(k.Surface).Border(theme.BorderWidth, k.Border).Radius(theme.PopupRadius).
 			Size(w-u*12, h-u*12).Padding(u * 3).Gap(u * 3).Label(core.Msg(c, "image_viewer.title"))
@@ -92,7 +92,7 @@ func ImageViewer(c *ui.Context, open *bool, index *int, images []ViewerImage) co
 			if tool("chevron-left", "image_viewer.prev", *index == 0) {
 				goTo(*index - 1)
 			}
-			stage := ui.Box(c).Grow(1).FillHeight().MinWidth(0).Focusable().AutoFocus().Clip().
+			stage := core.AutoFocus(c, ui.Box(c).Grow(1).FillHeight().MinWidth(0).Focusable()).Clip().
 				Radius(theme.ControlRadius).Background(k.Background).Role(ui.RoleImage).Label(img.Alt).Cursor(ui.CursorGrab)
 			internal.Ring(stage, k.Focus, theme.ControlRadius)
 			if dx, dy, ok := stage.Dragged(); ok {
@@ -143,7 +143,7 @@ func ImageViewer(c *ui.Context, open *bool, index *int, images []ViewerImage) co
 }
 
 // viewerKeys, anywhere in the panel, zooms with + - 0, switches with Left and Right, and pans with Shift+arrows.
-func viewerKeys(panel *ui.Element, st *viewerState, zoom func(float32), goTo func(int), at int) {
+func viewerKeys(panel ui.Element, st *viewerState, zoom func(float32), goTo func(int), at int) {
 	switch {
 	case panel.Shortcut(0, ui.KeyEqual), panel.Shortcut(ui.Shift, ui.KeyEqual):
 		zoom(viewerZoomStep)