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>

docs/components/461-settings-layout.md

@@ -16,7 +16,7 @@ view := func(c *ui.Context) {
 	core.Use(c, core.Settings{})
 	account.SettingsLayout(c, &page, pages, account.SettingsLayoutOptions{}, func(id string) {
 		account.SettingsSection(c, "Notifications", func() {
-			account.SettingsRow(c, "Sound", account.SettingsRowOptions{ID: "sound"}, func() *ui.Element {
+			account.SettingsRow(c, "Sound", account.SettingsRowOptions{ID: "sound"}, func() ui.Element {
 				return input.Switch(c, &sound, "", input.SwitchOptions{})
 			})
 		})
@@ -33,7 +33,7 @@ ui.Render(view, 400, 300, 1)
 | `pages` | `[]SettingsPage` | 必填 | ID、Label、Icon。 |
 | `Title` | `string` | 内置“Settings” | 导航标题。 |
 | `content` | `func(id string)` | 必填 | 构建分区内容。 |
-| `SettingsRow control` | `func() *ui.Element` | 必填 | 返回控件；行标签与说明关联到它。 |
+| `SettingsRow control` | `func() ui.Element` | 必填 | 返回控件；行标签与说明关联到它。 |
 | `SettingsRowOptions.ID` | `string` | "" | 供 SettingsSearch 跳转定位。 |
 
 ## 状态