Add samples for [R3Command] and interface events

[Skymly]

Summary

The Samples project needs examples demonstrating:

1. [R3Command] usage with CanExecute support
2. Interface-type events (e.g., INotifyPropertyChanged.FromEvents())

Current State

• Samples project has ObservableEvents examples
• No [R3Command] demonstrations
• No interface event demonstrations

Proposed Additions

1. R3CommandViewModel: Demonstrate basic command, async command, and CanExecute
2. InterfaceEventsViewModel: Show INotifyPropertyChanged.FromEvents() usage

Acceptance Criteria

• R3CommandViewModel with examples
• InterfaceEventsViewModel with INotifyPropertyChanged example
• Views updated to showcase new samples
• Samples compile and run correctly