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/volume_chart.go

@@ -74,6 +74,7 @@ func VolumeChart(c *ui.Context, candles []Candle, view *MarketView, opts VolumeC
 		return strings.Join([]string{cd.Time.Format("2006-01-02 15:04"), core.Msg(c, "candlestick_chart.volume") + " " + data.StatisticFormat(cd.Volume, 0),
 			core.TrendWord(c, core.TrendSign(cd.Close-cd.Open))}, " · ")
 	}
+	st := m.st0() // painting runs after the build, when Local is gone
 	spec.Draw = func(p *ui.Painter, r ui.Rect, active int) {
 		plot, band, ys, ticks, step := geom(r)
 		internal.AxisY(p, plot, ys, ticks, func(v float64) string { return volumeShort(v, step) }, size, k)
@@ -99,14 +100,14 @@ func VolumeChart(c *ui.Context, candles []Candle, view *MarketView, opts VolumeC
 				p.Fill(bar, col, 0)
 			}
 		}
-		m.st0().drawn = m.to - m.from
+		st.drawn = m.to - m.from
 		if at >= m.from && at < m.to {
 			x := internal.Snap(p, band.Center(at-m.from))
 			p.Line(x, plot.Y, x, plot.Y+plot.H, 1, k.TextMuted)
 		}
 	}
 	res, plot := ch.Build(spec)
-	if plot != nil && !plot.IsDisabled() {
+	if plot.Valid() && !plot.IsDisabled() {
 		room := m.room()
 		switch {
 		case plot.Shortcut(ui.Shift, ui.KeyLeft):