Add a Control Widget example

[nimatime]

Why this matters

Control Widgets are a natural Apple-native surface for quick actions, and they pair well with App Intents. A small example would help contributors and users see how Axint should grow here.

Scope

• Add an example that models a simple control-style widget action.
• Include source contract, generated Swift expectations, and Xcode integration notes.
• Document current limitations if the compiler surface is not yet complete.

Done when

• The example is present under examples/ with a README.
• It is linked from the examples index.
• Tests or docs checks cover what can be validated today.