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>

internal/layout.go

@@ -3,7 +3,7 @@ package internal
 import "github.com/egoist/mygo/ui"
 
 // Remeasure rebuilds the view when layout gives e another size than the w×h it was built for.
-func Remeasure(c *ui.Context, e *ui.Element, w, h float32) *ui.Element {
+func Remeasure(c *ui.Context, e ui.Element, w, h float32) ui.Element {
 	return e.DrawOver(func(_ *ui.Painter, r ui.Rect) {
 		if abs(r.W-w) > 0.5 || abs(r.H-h) > 0.5 {
 			c.Invalidate()

internal/navigation.go

@@ -8,7 +8,7 @@ import (
 )
 
 // Ease moves e's background, border and text color fg to their targets over d.
-func Ease(e *ui.Element, d time.Duration, fg ui.Color) *ui.Element {
+func Ease(e ui.Element, d time.Duration, fg ui.Color) ui.Element {
 	if d > 0 {
 		e.Transition(ui.ElementTransition{Colors: true, Duration: d})
 	}

internal/paint.go

@@ -16,7 +16,7 @@ func FocusRing(p *ui.Painter, r ui.Rect, radius float32, c ui.Color) {
 }
 
 // Ring replaces MyGo's ring on e with the MujicaUI ring.
-func Ring(e *ui.Element, c ui.Color, radius float32) *ui.Element {
+func Ring(e ui.Element, c ui.Color, radius float32) ui.Element {
 	return e.FocusRing(false).DrawOver(func(p *ui.Painter, r ui.Rect) {
 		if e.FocusVisible() {
 			FocusRing(p, r, radius, c)