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>

datetime/calendar_time_grid.go

@@ -60,7 +60,7 @@ func timeGridSlot(name string, slot int) int {
 }
 
 // timeGrid draws day columns and turns gestures into changes.
-func timeGrid(c *ui.Context, root *ui.Element, days []core.Date, events []CalendarEvent, cfg timeGridConfig, r *CalendarViewResult) {
+func timeGrid(c *ui.Context, root ui.Element, days []core.Date, events []CalendarEvent, cfg timeGridConfig, r *CalendarViewResult) {
 	k := core.Tokens(c)
 	d := core.Density(c)
 	hourH := d.ControlHeight() * 2
@@ -266,7 +266,7 @@ func timeChip(c *ui.Context, i int, pl timePlaced, ev CalendarEvent, cfg timeGri
 	if mine && g.kind == CalendarResize {
 		growY = float32(snap(g.dy)) * hourH / 60
 	}
-	box := ui.Box(c).Key(ev.ID).Absolute().Top(from + shiftY).Height(max(to-from+growY, theme.MinHitSize)).
+	box := ui.Box(c.Key(ev.ID)).Absolute().Top(from + shiftY).Height(max(to-from+growY, theme.MinHitSize)).
 		LeftPercent(100 * float32(pl.col) / float32(pl.cols)).WidthPercent(100 / float32(pl.cols)).Padding(1)
 	box.Children(func() {
 		chip := EventChip(c, ev, EventChipOptions{Location: cfg.loc, Hour12: cfg.hour12, Selected: mine, Inline: to-from < hourH*0.75}).Fill()
@@ -291,7 +291,7 @@ func timeChip(c *ui.Context, i int, pl timePlaced, ev CalendarEvent, cfg timeGri
 			}
 		}
 		for _, part := range []struct {
-			e    *ui.Element
+			e    ui.Element
 			kind CalendarChangeKind
 		}{{chip, CalendarMove}, {handle, CalendarResize}} {
 			dx, dy, ok := part.e.Dragged()