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>

data/collapsible.go

@@ -55,7 +55,7 @@ func Collapsible(c *ui.Context, open *bool, opts CollapsibleOptions, content fun
 				ui.Text(c, opts.Title).FontWeight(600).SingleLine()
 			}
 		})
-		if panel := collapsiblePanel(c, *open, prog, content); panel != nil {
+		if panel := collapsiblePanel(c, *open, prog, content); panel.Valid() {
 			panel.Padding(u*2, u*2, u*2, u*8).Gap(u * 2)
 		}
 	})
@@ -73,13 +73,13 @@ func collapsibleTarget(open bool) float32 {
 }
 
 // collapsiblePanel builds fn while open or animating, clipped to its progress.
-func collapsiblePanel(c *ui.Context, open bool, prog float32, fn func()) *ui.Element {
+func collapsiblePanel(c *ui.Context, open bool, prog float32, fn func()) ui.Element {
 	if !open && prog == 0 {
-		return nil
+		return ui.Element{}
 	}
 	// Closing content takes no input or focus while it shrinks away.
 	clip := ui.Column(c).Shrink(0).Disabled(!open)
-	var inner *ui.Element
+	var inner ui.Element
 	clip.Children(func() {
 		inner = ui.Column(c).Shrink(0)
 		inner.Children(fn)