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>

input/input_review_test.go

@@ -8,6 +8,7 @@ import (
 
 	"git.zacharyzhang.com/ZacharyZhang-NY/MujicaUI/core"
 	"git.zacharyzhang.com/ZacharyZhang-NY/MujicaUI/icons"
+	"git.zacharyzhang.com/ZacharyZhang-NY/MujicaUI/theme"
 )
 
 func TestButtonLoadingKeepsWidth(t *testing.T) {
@@ -165,9 +166,12 @@ func inputReviewStates(t *testing.T, name string, view func(c *ui.Context)) (pla
 		t.Fatalf("no %q", name)
 	}
 	plain = encode(t, tt.Image())
+	// Colors still fade under reduced motion; let each fade finish on the Tester clock.
 	tt.Move(r.X+r.W/2, r.Y+r.H/2)
+	tt.Advance(theme.StateDuration)
 	hover = encode(t, tt.Image())
 	tt.Press(r.X+r.W/2, r.Y+r.H/2)
+	tt.Advance(theme.StateDuration)
 	pressed = encode(t, tt.Image())
 	return
 }