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>

git/branch_selector.go

@@ -23,7 +23,7 @@ type BranchSelectorOptions struct {
 
 // BranchSelectorResult is a built BranchSelector and its events.
 type BranchSelectorResult struct {
-	Element *ui.Element
+	Element ui.Element
 	changed bool
 	create  string
 }
@@ -82,7 +82,7 @@ func BranchSelector(c *ui.Context, selected *string, branches []Branch, opts Bra
 		ui.Icon(c, icons.Must("chevrons-up-down")).Size(14, 14).TextColor(k.TextMuted)
 	})
 	b := tr.Bounds()
-	ui.PopoverBase(c, tr, &s.open, func(panel *ui.Element) {
+	ui.PopoverBase(c, tr, &s.open, func(panel ui.Element) {
 		core.OverlayStyle(c, panel, overlay.OverlayAnchored(c, panel, b))
 		panel.Margin(6, 0, 0, 0).Padding(u * 2).Gap(u * 2).Width(300).MaxHeight(overlay.PopoverRoom(c, b)).AlignItems(ui.Stretch)
 		var first *Branch
@@ -100,7 +100,7 @@ func BranchSelector(c *ui.Context, selected *string, branches []Branch, opts Bra
 		}
 		in := input.TextInput(c, &s.query, input.TextInputOptions{Placeholder: core.Msg(c, "branch_selector.search"), Label: core.Msg(c, "branch_selector.search"),
 			Leading: func() { ui.Icon(c, icons.Must("search")).Size(14, 14).TextColor(k.TextMuted) }})
-		in.Input.AutoFocus()
+		core.AutoFocus(c, in.Input)
 		if in.Changed() {
 			s.hl, s.reveal = 0, true
 		}