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>

devtools/service_status.go

@@ -35,7 +35,7 @@ type ServiceStatusOptions struct {
 }
 
 // ServiceStatus lists service health and an overall summary.
-func ServiceStatus(c *ui.Context, services []ServiceItem, opts ServiceStatusOptions) *ui.Element {
+func ServiceStatus(c *ui.Context, services []ServiceItem, opts ServiceStatusOptions) ui.Element {
 	if opts.MaxHeight < 0 {
 		panic("mujica: ServiceStatus MaxHeight is negative")
 	}
@@ -75,7 +75,7 @@ func ServiceStatus(c *ui.Context, services []ServiceItem, opts ServiceStatusOpti
 		agent.AgentRows(c, len(services), func(i int) {
 			s := services[i]
 			word := serviceWord(c, s.Health)
-			ui.Row(c).Key(i).Gap(u*3).AlignItems(ui.Center).MinHeight(core.Density(c).ControlHeight()).Padding(u, u*3).
+			ui.Row(c.Key(i)).Gap(u*3).AlignItems(ui.Center).MinHeight(core.Density(c).ControlHeight()).Padding(u, u*3).
 				Label(s.Name + ", " + word).Children(func() {
 				ui.Column(c).Grow(1).Shrink(1).Children(func() {
 					ui.Text(c, s.Name).SingleLine()