Todo shared

Code shared by examples/todo and examples/todo_skia, so the two apps differ only in their backend:

• ui/todo_view.slint — the list UI: struct TodoItem and component TodoView (todo-model, add-todo, toggle-todo, remove-done, and the edit field a test finds as TodoView::edit). Each app's ui/todo.slint imports it, wraps it in a window of its own and forwards the property and callbacks. It is not a build_runner source of this package: each app's generated wrapper embeds it (TodoApp.slintFiles), and examples/todo's AOT build compiles it.
• TodoStore and TodoEntry, the seed list and the AppBar title helper — framework-free, so the rules (trim-on-add, ignore-empty, bounds-checked toggle, drop-checked-on-remove) live here once.
• TodoExampleApp and TodoPageStateMixin — the Flutter page around the store: the callbacks, sync + rebuild, the Scaffold, the load error. Each app loads its backend and implements pushTodos, mapping TodoEntry to its generated TodoItem at the Slint boundary.
• package:todo_shared/testing.dart — testWrapperEmbedsCurrentSlint(TodoApp.slintSource, TodoApp.slintFiles), which checks that a wrapper's embedded copies still match ui/todo.slint and the todo_view.slint it imports. The shared file is the easy one to miss: editing it rebuilds neither app.

No Slint wire format lives here: the apps set todoModel and register onAddTodo/onToggleTodo/onRemoveDone through their generated wrappers. Depends on no app and no generated code. Tested under flutter test.