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/scroll_to_bottom_button.go

@@ -13,7 +13,7 @@ import (
 
 // ScrollToBottomButton shows while away is true, with the unread count; click it to return to the end.
 // While hidden it returns an empty element whose Clicked is false.
-func ScrollToBottomButton(c *ui.Context, away bool, unread int) *ui.Element {
+func ScrollToBottomButton(c *ui.Context, away bool, unread int) ui.Element {
 	if unread < 0 {
 		panic("mujica: ScrollToBottomButton unread must not be negative")
 	}
@@ -27,7 +27,7 @@ func ScrollToBottomButton(c *ui.Context, away bool, unread int) *ui.Element {
 		label = fmt.Sprintf(core.Msg(c, "scroll_to_bottom_button.unread"), unread)
 	}
 	wrap := ui.Row(c).Absolute().Left(0).Right(0).Bottom(u * 3).Justify(ui.Center).PassThrough()
-	var b *ui.Element
+	var b ui.Element
 	wrap.Children(func() {
 		b = ui.ButtonBase(c).MinHeight(theme.MinHitSize).MinWidth(theme.MinHitSize).Padding(0, u*3).Gap(u*2).
 			Radius(theme.MinHitSize/2).Label(label).Background(k.Surface).Border(theme.BorderWidth, k.ControlBorder).