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>

feedback/progress.go

@@ -33,7 +33,7 @@ type ProgressOptions struct {
 }
 
 // Progress shows work done, value 0 to 1; indeterminate ones hold still under reduced motion.
-func Progress(c *ui.Context, value float64, opts ProgressOptions) *ui.Element {
+func Progress(c *ui.Context, value float64, opts ProgressOptions) ui.Element {
 	if opts.Shape != ProgressLinear && opts.Shape != ProgressRing {
 		panic(fmt.Sprintf("mujica: unknown ProgressShape %d", int(opts.Shape)))
 	}
@@ -102,7 +102,7 @@ func Progress(c *ui.Context, value float64, opts ProgressOptions) *ui.Element {
 	return col
 }
 
-func progressRing(c *ui.Context, k theme.Tokens, u float32, acc float64, name, caption string, opts ProgressOptions, still bool) *ui.Element {
+func progressRing(c *ui.Context, k theme.Tokens, u float32, acc float64, name, caption string, opts ProgressOptions, still bool) ui.Element {
 	d := opts.Size
 	if d == 0 {
 		d = 48
@@ -157,7 +157,7 @@ func progressRing(c *ui.Context, k theme.Tokens, u float32, acc float64, name, c
 }
 
 // progressAnimate eases toward target; under reduced motion it lands there and keeps the state in step.
-func progressAnimate(e *ui.Element, target float32, still bool) float32 {
+func progressAnimate(e ui.Element, target float32, still bool) float32 {
 	if still {
 		// A changed target lands at once under reduced motion; the detour stops a running transition.
 		e.Animate("progress.value", target+1, theme.StateDuration)
@@ -193,7 +193,7 @@ func meterLevel(v float64, l *ui.MeterLevels) int {
 }
 
 // Meter shows a level within a range with threshold ticks and a level word, not color alone.
-func Meter(c *ui.Context, value float64, opts MeterOptions) *ui.Element {
+func Meter(c *ui.Context, value float64, opts MeterOptions) ui.Element {
 	lo, hi := opts.Min, opts.Max
 	if lo == 0 && hi == 0 {
 		hi = 1