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>

AGENTS.md

@@ -9,11 +9,11 @@ Planned: once 001–500 land, the root package splits into category sub-packages
 | Item | Value |
 | --- | --- |
 | Go | 1.27.1 (MyGo's minimum) |
-| MyGo | `github.com/egoist/mygo v0.2.10` (commit 459b512), pinned in go.mod |
+| MyGo | `github.com/egoist/mygo v0.5.5` (commit 4d8673c), pinned in go.mod |
 | Icons | Lucide 1.52.0, ISC, copied into `icons/svg` |
 | Web mirror | `examples/gallery -web :8080`: Go stdlib server drives the Gallery through `ui.NewTester` and streams frames to a canvas. The browser shows pixels; Go runs the components. |
 
-Read MyGo source before using an API: `go env GOMODCACHE`/github.com/egoist/mygo@v0.2.10 (`ui/*.go`, `docs/ui/*.md`). Never guess.
+Read MyGo source before using an API: `go env GOMODCACHE`/github.com/egoist/mygo@v0.5.5 (`ui/*.go`, `docs/ui/*.md`, migration guide `docs/ui/migration.md`). Never guess.
 
 ## Layout
 
@@ -36,8 +36,8 @@ Gallery categories map to packages: Inputs, Choices, Forms, Essentials -> input;
 
 ## Component rules
 
-- Signature: `func Name(c *ui.Context, value *T, opts NameOptions) *ui.Element`. Composites may return a named struct.
-- Result structs expose the element as a named field `Element *ui.Element`, never embedded; element plus changed is `ChoiceResult`.
+- Signature: `func Name(c *ui.Context, value *T, opts NameOptions) ui.Element`. Composites may return a named struct.
+- Result structs expose the element as a named field `Element ui.Element`, never embedded; element plus changed is `ChoiceResult`.
 - Shared helpers: `iconAction` (borderless icon button), `overlayStyle(c, panel, overlayEnter(c, panel))` (popup surface and entry fade), `internal.RuleDiamond` / `internal.Frame` (ornaments), `colorFade` (state color transition).
 - Zero `Options` gives the default look. No `map[string]any` config.
 - Explicit font sizes: `FontSize(c, theme.CaptionSize)`; it applies the desktop text scale.
@@ -50,7 +50,9 @@ Gallery categories map to packages: Inputs, Choices, Forms, Essentials -> input;
 - Durations: `core.Motion(c, theme.StateDuration)`; popups use `theme.PopupDuration`.
 - Disabled blocks input before handling it (`Disabled(true)`); MyGo dims disabled elements, do not add opacity.
 - Copy shown by a component comes from `core.Msg(c, key)`; business copy comes from the caller.
-- No cross-frame `*ui.Element` caching, no goroutines touching elements.
+- `ui.Element` is a handle valid only during its build pass: no cross-frame caching, no goroutines touching elements, no element methods inside `HandleInput` or other callbacks (read state such as `IsDisabled` into a local during the build). Keep a `ui.Handle` for persistent identity and `c.Services()` for clipboard or URLs from callbacks. The absent element is `ui.Element{}`; test it with `.Valid()`.
+- Overlay content takes initial focus with `core.AutoFocus(c, e)`, not `e.AutoFocus()`: MyGo v0.5.5 records an overlay's opener after its content builds, so same-frame autofocus loses focus restore on close.
+- Tests run on the Tester's virtual clock: `tt.Advance(d)` and `tt.Now()`, never `time.Sleep` or `time.Now()`; read element state inside the view.
 - Fail fast: panic on programmer errors (bad options); never swallow errors.
 - Log state transitions in composites with `slog.Debug("mujica: <component> <event>", ...)`. Never log passwords.
 - Files under 500 lines. Comments: one line, only when needed.