Add EventSource trait abstraction for testable event injection

[bug-ops]

Parent: #542

Problem

EventReader is hardwired to crossterm, making it impossible to inject synthetic events in tests. The tui_loop cannot be exercised without a real terminal.

Requirements

• Extract EventSource trait from EventReader
• Implement CrosstermEventSource for production use
• Implement MockEventSource accepting Vec<AppEvent> or mpsc::Receiver<AppEvent>
• Make tui_loop generic over EventSource

Acceptance criteria

• tui_loop can run in tests with synthetic events without a real terminal
• No behavioral changes to production code path
• All existing tests pass