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>

core/overlay_motion.go

@@ -9,7 +9,7 @@ import (
 )
 
 // OverlayEnter returns how far, 0 to 1, the popup born with e has entered.
-func OverlayEnter(c *ui.Context, e *ui.Element) float32 {
+func OverlayEnter(c *ui.Context, e ui.Element) float32 {
 	born := ui.Local(e, "mujica.overlay.born", func() time.Time { return c.Now() })
 	d := Motion(c, theme.PopupDuration)
 	if d == 0 {
@@ -24,7 +24,7 @@ func OverlayEnter(c *ui.Context, e *ui.Element) float32 {
 }
 
 // OverlayStyle gives a popup panel the surface, edge and shadow of the theme.
-func OverlayStyle(c *ui.Context, panel *ui.Element, p float32) {
+func OverlayStyle(c *ui.Context, panel ui.Element, p float32) {
 	k := Tokens(c)
 	panel.Background(k.Surface).Border(theme.BorderWidth, k.Border).Radius(theme.PopupRadius).
 		TextColor(k.Text).Opacity(p).Shadow(0, 6, 18, 0, ui.RGBA(0, 0, 0, 0.16*p))