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/input_group.go

@@ -37,9 +37,9 @@ type InputGroupOptions struct {
 
 // InputGroupParts are the parts of an InputGroup.
 type InputGroupParts struct {
-	Group *ui.Element
+	Group ui.Element
 	// Input takes the focus; it is selectable text when ReadOnly.
-	Input *ui.Element
+	Input ui.Element
 	// Clicked is the index in Buttons of the button clicked, or -1.
 	Clicked int
 	locked  bool
@@ -77,7 +77,7 @@ func InputGroup(c *ui.Context, value *string, opts InputGroupOptions) InputGroup
 		n++
 	}
 	seg := 0
-	segment := func(e *ui.Element) {
+	segment := func(e ui.Element) {
 		var left, right float32
 		if seg == 0 {
 			left = r
@@ -180,15 +180,15 @@ type inputGroupLock struct {
 }
 
 // inputGroupEditor builds the input, or plain selectable text while locked, so no edit command runs.
-func inputGroupEditor(c *ui.Context, box *ui.Element, value *string, locked bool) *ui.Element {
+func inputGroupEditor(c *ui.Context, box ui.Element, value *string, locked bool) ui.Element {
 	lk := ui.Local(box, "lock", func() inputGroupLock { return inputGroupLock{} })
 	if lk.locked && !locked {
 		// A new identity drops the edits queued in the old editor while locked.
 		lk.gen++
 	}
 	lk.locked = locked
-	var in *ui.Element
-	ui.Row(c).Key(lk.gen).FillWidth().MinWidth(0).Children(func() {
+	var in ui.Element
+	ui.Row(c.Key(lk.gen)).FillWidth().MinWidth(0).Children(func() {
 		if locked {
 			in = ui.Text(c, *value).SingleLine().Selectable()
 		} else {