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/event_popover.go

@@ -26,7 +26,7 @@ type EventPopoverOptions struct {
 
 // EventPopoverResult is the popover and the action the user chose.
 type EventPopoverResult struct {
-	Element *ui.Element
+	Element ui.Element
 	edit    bool
 	del     bool
 }
@@ -38,7 +38,7 @@ func (r EventPopoverResult) Edit() bool { return r.edit }
 func (r EventPopoverResult) Delete() bool { return r.del }
 
 // EventPopover summarizes an event beside anchor while *open.
-func EventPopover(c *ui.Context, anchor *ui.Element, open *bool, ev CalendarEvent, opts EventPopoverOptions) EventPopoverResult {
+func EventPopover(c *ui.Context, anchor ui.Element, open *bool, ev CalendarEvent, opts EventPopoverOptions) EventPopoverResult {
 	loc := eventLocation("EventPopover", opts.Location)
 	k := core.Tokens(c)
 	d := core.Density(c)
@@ -47,7 +47,7 @@ func EventPopover(c *ui.Context, anchor *ui.Element, open *bool, ev CalendarEven
 		slog.Debug("mujica: event_popover close", "disabled", true)
 		*open = false
 	}
-	r.Element = ui.PopoverBase(c, anchor, open, func(panel *ui.Element) {
+	r.Element = ui.PopoverBase(c, anchor, open, func(panel ui.Element) {
 		core.OverlayStyle(c, panel, core.OverlayEnter(c, panel))
 		ww, wh := c.Size()
 		panel.Margin(d.Unit(), 0, 0, 0).Padding(d.Unit() * 4).Gap(d.Unit() * 2).Width(min(d.ControlHeight()*9, ww-16)).MaxHeight(wh - 16).