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>

chat/source_card.go

@@ -13,7 +13,7 @@ import (
 
 // SourceCard displays source metadata; Clicked requests opening.
 // Selection and an edge mark identify active or hovered sources.
-func SourceCard(c *ui.Context, n int, src Source, st *CitationState) *ui.Element {
+func SourceCard(c *ui.Context, n int, src Source, st *CitationState) ui.Element {
 	if n < 1 || st == nil || src.Title == "" || src.URL == "" {
 		panic("mujica: SourceCard needs n >= 1, a state and a Source with Title and URL")
 	}
@@ -23,8 +23,7 @@ func SourceCard(c *ui.Context, n int, src Source, st *CitationState) *ui.Element
 		Label(fmt.Sprintf(core.Msg(c, "source_card.open"), src.Title)).Tooltip(src.URL).Transition(core.ColorFade)
 	internal.Ring(b, k.Focus, theme.CardRadius)
 	st.track(c, b, n)
-	frame := c.Now()
-	b.Draw(func(_ *ui.Painter, _ ui.Rect) { st.expire(c, frame) })
+	b.Draw(func(_ *ui.Painter, _ ui.Rect) { st.expire(c) })
 	switch {
 	case st.lit(n):
 		b.Background(k.Selection).Border(theme.BorderWidth, k.Border).BorderWidth(theme.BorderWidth, theme.BorderWidth, theme.BorderWidth, 3).