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/project_list.go

@@ -35,7 +35,7 @@ type ProjectListState struct {
 
 // ProjectListView is a built ProjectList and this frame's events.
 type ProjectListView struct {
-	Element *ui.Element
+	Element ui.Element
 	// Changed reports a newly chosen project, in State.Selected.
 	Changed bool
 	// Opened is a conversation the user opened, empty for none.
@@ -64,7 +64,7 @@ func ProjectList(c *ui.Context, s *ProjectListState, projects []ChatProject, con
 			}
 			seen[p.Folder] = true
 			open := !s.closed[p.Folder]
-			ui.Column(c).Key("folder:" + p.Folder).Children(func() { projectListFolder(c, p.Folder, &open) })
+			ui.Column(c.Key("folder:" + p.Folder)).Children(func() { projectListFolder(c, p.Folder, &open) })
 			s.closed[p.Folder] = !open
 			if !open {
 				continue
@@ -94,7 +94,7 @@ func ProjectList(c *ui.Context, s *ProjectListState, projects []ChatProject, con
 }
 
 // projectListFolder is a folder heading on a CollapsibleBase trigger.
-func projectListFolder(c *ui.Context, title string, open *bool) *ui.Element {
+func projectListFolder(c *ui.Context, title string, open *bool) ui.Element {
 	k := core.Tokens(c)
 	d := core.Density(c)
 	parts := ui.CollapsibleBase(c, open)
@@ -121,8 +121,8 @@ func projectListProject(c *ui.Context, s *ProjectListState, p ChatProject, all [
 	d := core.Density(c)
 	convs := conversations(p.ID)
 	chosen := s.Selected == p.ID
-	var b *ui.Element
-	line := ui.Row(c).Key("project:" + p.ID).AlignItems(ui.Center)
+	var b ui.Element
+	line := ui.Row(c.Key("project:" + p.ID)).AlignItems(ui.Center)
 	line.Children(func() { b = ui.ButtonBase(c).Grow(1).MinWidth(0).MinHeight(d.ControlHeight()) })
 	b.Padding(0, d.Unit()*2, 0, d.Unit()*2+indent).Padding(0, d.Unit()*2, 0, d.Unit()*2+indent).
 		Gap(d.Unit() * 2).Justify(ui.Start).Radius(theme.ControlRadius).Role(ui.RoleTreeItem).Label(p.Name).Selected(chosen)
@@ -185,7 +185,7 @@ func projectListProject(c *ui.Context, s *ProjectListState, p ChatProject, all [
 func projectListConversation(c *ui.Context, conv ChatConversation, home ChatProject, all []ChatProject, indent float32, v *ProjectListView) {
 	k := core.Tokens(c)
 	d := core.Density(c)
-	ui.Row(c).Key("conv:" + conv.ID).AlignItems(ui.Center).Children(func() {
+	ui.Row(c.Key("conv:" + conv.ID)).AlignItems(ui.Center).Children(func() {
 		b := ui.ButtonBase(c).Grow(1).MinWidth(0).MinHeight(d.ControlHeight()).Padding(0, d.Unit()*2, 0, d.Unit()*2+indent).
 			Justify(ui.Start).Radius(theme.ControlRadius).Label(conv.Title).Drag(ConversationDrag{ID: conv.ID})
 		internal.Ring(b, k.Focus, theme.ControlRadius)