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>

input/dock_panel.go

@@ -124,7 +124,7 @@ func dockPanelZone(c *ui.Context, k theme.Tokens, u float32, layout *DockLayout,
 	if len(ids) == 0 {
 		return false
 	}
-	box := ui.Column(c).Key(z).MinWidth(0).MinHeight(0).Background(k.Surface).Clip().Role(ui.RoleGroup).
+	box := ui.Column(c.Key(z)).MinWidth(0).MinHeight(0).Background(k.Surface).Clip().Role(ui.RoleGroup).
 		Label(core.Msg(c, fmt.Sprintf("dock_panel.zone%d", z)))
 	switch z {
 	case DockLeft, DockRight:
@@ -173,7 +173,7 @@ func dockPanelZone(c *ui.Context, k theme.Tokens, u float32, layout *DockLayout,
 // dockPanelEdges shows drop strips on the four edges while a tab is dragged.
 func dockPanelEdges(c *ui.Context, k theme.Tokens, u float32, move func(string, DockZone)) {
 	for _, z := range []DockZone{DockLeft, DockRight, DockBottom, DockCenter} {
-		e := ui.Box(c).Key(fmt.Sprint("edge", z)).Absolute().Role(ui.RoleNone)
+		e := ui.Box(c.Key(fmt.Sprint("edge", z))).Absolute().Role(ui.RoleNone)
 		switch z {
 		case DockLeft:
 			e.Left(0).Top(0).Bottom(0).Width(12 * u)