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/042-form-field.md

@@ -12,7 +12,7 @@
 email := ""
 view := func(c *ui.Context) {
 	core.Use(c, core.Settings{})
-	input.FormField(c, "Email", input.FormFieldOptions{Required: true, Description: "For receipts."}, func() *ui.Element {
+	input.FormField(c, "Email", input.FormFieldOptions{Required: true, Description: "For receipts."}, func() ui.Element {
 		return input.InputGroup(c, &email, input.InputGroupOptions{}).Input
 	})
 }
@@ -28,7 +28,7 @@ ui.Render(view, 400, 300, 1)
 | `FormFieldOptions.Description` | `string` | "" | 控件下方的说明。 |
 | `FormFieldOptions.Error` | `string` | "" | 控件下方的错误，并把控件标记为无效。 |
 | `FormFieldOptions.Horizontal` | `bool` | false | 标签在左、控件在右。 |
-| `control` | `func() *ui.Element` | 必填 | 构建控件并返回要关联的可聚焦元素（如 `InputGroupParts.Input`）；返回 nil 时 panic。 |
+| `control` | `func() ui.Element` | 必填 | 构建控件并返回要关联的可聚焦元素（如 `InputGroupParts.Input`）；返回零值 `ui.Element{}` 时 panic。 |
 
 ## 状态