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>

code/terminal_tabs.go

@@ -30,7 +30,7 @@ type TerminalTabsOptions struct {
 
 // TerminalTabsResult is TerminalTabs and what happened in this frame.
 type TerminalTabsResult struct {
-	Element *ui.Element
+	Element ui.Element
 	// Renamed and Closed hold a tab ID.
 	Changed bool
 	Renamed string
@@ -126,7 +126,7 @@ func TerminalTabs(c *ui.Context, tabs *[]TerminalTab, active *string, opts Termi
 }
 
 // terminalTabsTab builds one tab with title and close.
-func terminalTabsTab(c *ui.Context, tab *ui.Element, t TerminalTab, on bool, st *terminalTabsState, rename func(string), askClose func(TerminalTab), disabled bool) {
+func terminalTabsTab(c *ui.Context, tab ui.Element, t TerminalTab, on bool, st *terminalTabsState, rename func(string), askClose func(TerminalTab), disabled bool) {
 	k := core.Tokens(c)
 	d := core.Density(c)
 	u := d.Unit()
@@ -162,7 +162,7 @@ func terminalTabsTab(c *ui.Context, tab *ui.Element, t TerminalTab, on bool, st
 		if st.editing == t.ID {
 			in := input.TextInput(c, &st.draft, input.TextInputOptions{Label: core.Msg(c, "terminal_tabs.rename")})
 			in.Element.Width(u * 36)
-			in.Input.AutoFocus()
+			core.AutoFocus(c, in.Input)
 			switch {
 			case in.Input.Shortcut(0, ui.KeyEscape):
 				st.editing = ""