Commanding in Reactor

Use Command when an action shows up in multiple surfaces (toolbar + menu + context menu), needs a keyboard shortcut, or needs CanExecute disabling. Use a bare Action for one-off button clicks with no reuse.

Controlled prop note: command examples may host controlled inputs such as TextBox(text, onChange). Factory call sites stay plain-valued, but direct reads from migrated element records are Optional<T>; see migration/050-optional-t.md.

Command record