`toolbar_of` builds the document and hands back the controls it was asked for,
so a test no longer creates a `Document`, threads `&mut Document` through
`with_reactive_scope`, and sets a root by hand. The button helpers are
components with an `on_click` prop, which replaces `counting_button` and its
`set_button_on_click` patching.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017W8YfdzDmZcR8A76jRjvZY