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/resizable_panel_group.go

@@ -32,7 +32,7 @@ type ResizablePanelGroupOptions struct {
 
 // ResizablePanelGroupResult reports a group's changes: Resized while moving, Committed when a change is done, to save.
 type ResizablePanelGroupResult struct {
-	Element   *ui.Element
+	Element   ui.Element
 	Resized   bool
 	Committed bool
 }
@@ -88,7 +88,7 @@ func ResizablePanelGroup(c *ui.Context, sizes *[]float32, panels []ResizablePane
 	for i, p := range panels {
 		mins[i] = cmp.Or(p.Min, 10*u)
 	}
-	along := func(e *ui.Element, v float32) *ui.Element {
+	along := func(e ui.Element, v float32) ui.Element {
 		if opts.Vertical {
 			return e.Height(v).FillWidth()
 		}
@@ -109,7 +109,7 @@ func ResizablePanelGroup(c *ui.Context, sizes *[]float32, panels []ResizablePane
 			if (*sizes)[i] == 0 {
 				continue
 			}
-			pane := ui.Box(c).Key(i).Shrink(0).Clip().Role(ui.RoleGroup).Label(p.Label)
+			pane := ui.Box(c.Key(i)).Shrink(0).Clip().Role(ui.RoleGroup).Label(p.Label)
 			if avail > 0 {
 				along(pane, (*sizes)[i]*avail)
 			} else {
@@ -128,7 +128,7 @@ func ResizablePanelGroup(c *ui.Context, sizes *[]float32, panels []ResizablePane
 // resizablePanelDivider builds the divider after panel i, stepped with arrows, Enter collapsing; it returns the grip's builder.
 func resizablePanelDivider(c *ui.Context, k theme.Tokens, u float32, st *resizablePanelState, sizes *[]float32, panels []ResizablePanel, mins []float32, avail float32, i int, vertical, disabled bool, res *ResizablePanelGroupResult) func() {
 	s := *sizes
-	div := ui.Box(c).Key(fmt.Sprint("div", i)).Shrink(0).Focusable().FocusRing(false).Role(ui.RoleSplitter).
+	div := ui.Box(c.Key(fmt.Sprint("div", i))).Shrink(0).Focusable().FocusRing(false).Role(ui.RoleSplitter).
 		Label(fmt.Sprintf(core.Msg(c, "resizable_panel_group.divider"), panels[i].Label, panels[i+1].Label))
 	if vertical {
 		div.Height(theme.BorderWidth).FillWidth()
@@ -202,7 +202,7 @@ func resizablePanelDivider(c *ui.Context, k theme.Tokens, u float32, st *resizab
 		div.Description(core.Msg(c, "split_pane.collapsed"))
 	}
 	return func() {
-		grip := ui.Box(c).Key(fmt.Sprint("grip", i)).Absolute().Cursor(ui.CursorResizeEW)
+		grip := ui.Box(c.Key(fmt.Sprint("grip", i))).Absolute().Cursor(ui.CursorResizeEW)
 		if vertical {
 			grip.Cursor(ui.CursorResizeNS)
 		}
@@ -231,7 +231,7 @@ func resizablePanelDivider(c *ui.Context, k theme.Tokens, u float32, st *resizab
 }
 
 // resizablePanelGrip places the grip over divider i and draws the divider.
-func resizablePanelGrip(c *ui.Context, k theme.Tokens, u float32, s []float32, avail float32, i int, vertical bool, grip, div *ui.Element, held bool) {
+func resizablePanelGrip(c *ui.Context, k theme.Tokens, u float32, s []float32, avail float32, i int, vertical bool, grip, div ui.Element, held bool) {
 	// The grip is wider than the 1 DIP line, centered on the boundary.
 	var at float32
 	for _, f := range s[:i+1] {