Complete Async/Await Support (Futures)

[chavic]

Problem:
Basic async infrastructure exists but needs completion. Current implementation fails due to trait compatibility issues.

Affected Fixture:

• dart_async - Fails with "trait SayAfterUdlTrait is not dyn compatible"

Acceptance Criteria:

• Fix dyn compatibility issues with async traits
• Support async fn in trait definitions
• Generate proper Dart Future<T> types
• Handle async error propagation
• Support async callbacks
• dart_async fixture passes all tests

Technical Implementation:

• Resolve E0038 dyn compatibility errors
• Update trait object generation for async methods
• Ensure proper Future handling in Dart

[chavic]

should be closed after #102 is merged