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/model_selector.go

@@ -44,7 +44,7 @@ func ModelSelector(c *ui.Context, value *string, models []ModelOption, opts Mode
 		}
 	}
 	k, u := core.Tokens(c), core.Density(c).Unit()
-	var tr *ui.Element
+	var tr ui.Element
 	ui.Row(c).Shrink(0).Disabled(opts.Disabled).Children(func() { tr = ui.ButtonBase(c).Role(ui.RolePopUpButton) })
 	st := ui.Local(tr, "model-selector", func() modelSelectorState { return modelSelectorState{hi: -1, lastHi: -1} })
 	disabled := tr.IsDisabled()
@@ -120,7 +120,7 @@ func ModelSelector(c *ui.Context, value *string, models []ModelOption, opts Mode
 	})
 	if st.open {
 		ww, wh := c.Size()
-		ui.PopoverBase(c, tr, &st.open, func(panel *ui.Element) {
+		ui.PopoverBase(c, tr, &st.open, func(panel ui.Element) {
 			core.OverlayStyle(c, panel, overlay.OverlayAnchored(c, panel, tr.Bounds()))
 			a := tr.Bounds()
 			room := max(a.Y, wh-a.Y-a.H) - 8