Add dedicated annotations package to stac_core with @StacScreen annotation for marking methods that return StacWidget instances.

- Create new `packages/stac_core/lib/annotations/` directory
- Add `stac_screen.dart` with @StacScreen annotation class
- Add `annotations.dart` export file for the annotations package
- Update `core.dart` to export annotations package

```dart
import 'package:stac_core/stac_core.dart';

@StacScreen('home')
StacWidget buildHomeScreen() => StacWidget();
```

Author: Potatomonsta

packages/stac_core/lib/annotations/annotations.dart

@@ -0,0 +1,3 @@
+library;
+
+export 'stac_screen.dart';

packages/stac_core/lib/annotations/stac_screen.dart

@@ -0,0 +1,20 @@
+/// Annotation to mark methods that return StacWidget instances.
+///
+/// This annotation is used to identify screen-level widgets in the Stac framework.
+/// Methods that return StacWidget should be annotated with this to indicate
+/// they represent screen definitions.
+///
+/// Example usage:
+/// ```dart
+/// @StacScreen('home')
+/// StacWidget buildHomeScreen() {
+///   return StacWidget(jsonData: {'type': 'scaffold', 'body': '...'});
+/// }
+/// ```
+class StacScreen {
+  /// Creates a [StacScreen] annotation with the given screen name.
+  const StacScreen(this.screenName);
+
+  /// The name identifier for this screen.
+  final String screenName;
+}

packages/stac_core/lib/core/core.dart

@@ -2,3 +2,4 @@ library;
 
 export 'stac_action.dart';
 export 'stac_widget.dart';
+export '../annotations/annotations.dart';

packages/stac_core/lib/stac_core.dart

@@ -1,6 +1,7 @@
 library;
 
 export 'actions/actions.dart';
+export 'annotations/annotations.dart';
 export 'core/core.dart';
 export 'foundation/foundation.dart';
 export 'widgets/widgets.dart';