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/tree_select.go

@@ -94,7 +94,7 @@ func TreeSelect(c *ui.Context, selected *[]string, nodes []TreeSelectNode, opts
 			MultiSelectTags(c, selected, options, MultiSelectOptions{MaxTags: opts.MaxTags, Disabled: opts.Disabled})
 		}
 	})
-	ui.PopoverBase(c, tr, &st.open, func(panel *ui.Element) {
+	ui.PopoverBase(c, tr, &st.open, func(panel ui.Element) {
 		SelectPanel(c, panel)
 		panel.MinWidth(max(tr.Bounds().W, 260))
 		ui.Column(c).Gap(4).Children(func() {
@@ -219,7 +219,7 @@ func treeSelectFirstChild(n *TreeSelectNode, shown map[string]bool) string {
 }
 
 // treeSelectDescribe says what MyGo cannot set on custom rows (ui/element.go:282).
-func treeSelectDescribe(c *ui.Context, b *ui.Element, branch, open bool, state internal.CheckState) {
+func treeSelectDescribe(c *ui.Context, b ui.Element, branch, open bool, state internal.CheckState) {
 	var parts []string
 	if branch {
 		parts = append(parts, core.Msg(c, map[bool]string{true: "tree_select.expanded", false: "tree_select.collapsed"}[open]))
@@ -238,7 +238,7 @@ func treeSelectRowView(c *ui.Context, st *treeSelectState, selected *[]string, r
 	n := r.node
 	branch := len(n.Children) > 0
 	open := branch && (st.expanded[n.ID] || strings.TrimSpace(st.query) != "")
-	ui.Row(c).Key(n.ID).AlignItems(ui.Center).Gap(2).Padding(0, 0, 0, float32(r.depth)*18).Children(func() {
+	ui.Row(c.Key(n.ID)).AlignItems(ui.Center).Gap(2).Padding(0, 0, 0, float32(r.depth)*18).Children(func() {
 		arrow := ui.Box(c).Size(theme.MinHitSize, theme.MinHitSize).Shrink(0).Radius(theme.ControlRadius)
 		if branch {
 			arrow.Role(ui.RoleButton).Label(fmt.Sprintf(core.Msg(c, map[bool]string{true: "tree_select.collapse", false: "tree_select.expand"}[open]), n.Label))
@@ -255,7 +255,7 @@ func treeSelectRowView(c *ui.Context, st *treeSelectState, selected *[]string, r
 			})
 		}
 		state := treeSelectCheck(n, *selected, opts)
-		var b *ui.Element
+		var b ui.Element
 		if opts.Multiple {
 			on := state == internal.Checked
 			b = MultiSelectCheck(c, k, &on, state, n.Label, n.Disabled)