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>

layout/grid.go

@@ -19,7 +19,7 @@ type GridOptions struct {
 }
 
 // Grid lays children out in columns and rows; GridCell places and spans them.
-func Grid(c *ui.Context, opts GridOptions, children func()) *ui.Element {
+func Grid(c *ui.Context, opts GridOptions, children func()) ui.Element {
 	if children == nil {
 		panic("mujica: Grid needs children")
 	}
@@ -48,7 +48,7 @@ type GridCellOptions struct {
 }
 
 // GridCell is one child of a Grid, placed and spanned by opts; build makes one element, stretched over the cell.
-func GridCell(c *ui.Context, opts GridCellOptions, build func()) *ui.Element {
+func GridCell(c *ui.Context, opts GridCellOptions, build func()) ui.Element {
 	if build == nil {
 		panic("mujica: GridCell needs build")
 	}
@@ -59,7 +59,7 @@ func GridCell(c *ui.Context, opts GridCellOptions, build func()) *ui.Element {
 		panic("mujica: GridCell spans must be positive, or -1 for the rest")
 	}
 	// A one-track grid stretches its child over the whole cell, as the grid does its own children.
-	e := ui.Grid(c).Columns(1).Rows(1)
+	e := ui.Grid(c).Columns(1).GridRows(1)
 	if opts.Column > 0 {
 		e.ColumnStart(opts.Column)
 	}