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>

chart/chart_brush.go

@@ -94,7 +94,7 @@ func ChartBrush(c *ui.Context, value *[2]float64, opts ChartBrushOptions) core.C
 		track.Draw(func(p *ui.Painter, r ui.Rect) { brushPaint(p, r, *value, opts, k, preview) })
 		track.Children(func() {
 			x0, x1 := at(value[0]), at(value[1])
-			end := func(i int, x float32) *ui.Element {
+			end := func(i int, x float32) ui.Element {
 				e := ui.Box(c).Absolute().Left(x-theme.MinHitSize/2).Top(0).Size(theme.MinHitSize, h).Focusable().
 					Role(ui.RoleSlider).Label(fmt.Sprintf(core.Msg(c, []string{"chart_brush.start", "chart_brush.end"}[i]), val(value[i])))
 				internal.Ring(e, k.Focus, theme.ControlRadius)
@@ -104,7 +104,7 @@ func ChartBrush(c *ui.Context, value *[2]float64, opts ChartBrushOptions) core.C
 				})
 			}
 			// Tab order runs start, window, end.
-			ends := [2]*ui.Element{end(0, x0)}
+			ends := [2]ui.Element{end(0, x0)}
 			window := ui.Box(c).Absolute().Left(x0 + theme.MinHitSize/2).Top(0).Width(max(x1-x0-theme.MinHitSize, theme.MinHitSize)).Height(h).Focusable().
 				Role(ui.RoleGroup).Label(fmt.Sprintf(core.Msg(c, "chart_brush.window"), opts.Label, val(value[0]), val(value[1])))
 			internal.Ring(window, k.Focus, theme.ControlRadius)
@@ -149,7 +149,7 @@ func ChartBrush(c *ui.Context, value *[2]float64, opts ChartBrushOptions) core.C
 }
 
 // brushKey handles arrow, Page, Home and End keys.
-func brushKey(e *ui.Element, v float64, spec input.NumberInputSpec) (float64, bool) {
+func brushKey(e ui.Element, v float64, spec input.NumberInputSpec) (float64, bool) {
 	if e.IsDisabled() {
 		return v, false
 	}
@@ -171,10 +171,10 @@ func brushKey(e *ui.Element, v float64, spec input.NumberInputSpec) (float64, bo
 }
 
 // brushPointer drags handles, the window, or a new range.
-func brushPointer(track, window *ui.Element, ends [2]*ui.Element, drag *brushDrag, value *[2]float64, from func(float32) float64, set, move func(lo, hi float64)) {
+func brushPointer(track, window ui.Element, ends [2]ui.Element, drag *brushDrag, value *[2]float64, from func(float32) float64, set, move func(lo, hi float64)) {
 	px, _, _ := track.PointerPosition()
 	at := from(px)
-	parts := []*ui.Element{ends[0], ends[1], window, track}
+	parts := []ui.Element{ends[0], ends[1], window, track}
 	held := -1
 	for i, e := range parts {
 		if _, _, ok := e.Dragged(); ok && held < 0 {