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/candlestick_chart_test.go

@@ -33,13 +33,12 @@ func mktState0(r ChartResult) *mktState {
 func TestCandlestickChartDrawsOnlyVisible(t *testing.T) {
 	candles := sampleCandles(100000)
 	var view MarketView
-	var res ChartResult
+	var st *mktState
 	tt := ui.NewTester(func(c *ui.Context) {
 		core.Use(c, core.Settings{})
-		res = CandlestickChart(c, candles, &view, CandlestickOptions{Frame: ChartFrame{Label: "BTC"}})
+		st = mktState0(CandlestickChart(c, candles, &view, CandlestickOptions{Frame: ChartFrame{Label: "BTC"}}))
 	}, 900, 400)
 	tt.Frame()
-	st := mktState0(res)
 	if st.drawn != 120 {
 		t.Fatalf("drew %d candles of 100000", st.drawn)
 	}