feat: Add signals_nocterm package for Nocterm TUI framework integration

[Norbert515]

Summary

This PR adds a new signals_nocterm package that integrates the signals reactive state management library with the Nocterm terminal UI framework.

Closes #442

Features

• SignalsMixin: Mixin for State classes to automatically manage signal lifecycle (create signals, computed values, and effects that are automatically disposed)
• Watch: Component for fine-grained rebuilds when signals change
• WatchBuilder: Builder pattern for watching a single signal
• watchSignal: Extension and function to watch signals in any build method

Implementation Details

The implementation follows the same patterns as signals_flutter to provide a familiar API for users of both libraries. Key adaptations for Nocterm:

• Uses Nocterm's SchedulerBinding instead of Flutter's for frame scheduling
• Uses Nocterm's Element and BuildContext instead of Flutter's
• Integrates with Nocterm's component lifecycle (State<T>, StatefulComponent, etc.)

Example Usage

class Counter extends StatefulComponent {
  const Counter({super.key});

  @override
  State<Counter> createState() => _CounterState();
}

class _CounterState extends State<Counter> with SignalsMixin {
  late final _count = createSignal(0);
  late final _doubled = createComputed(() => _count() * 2);

  @override
  void initState() {
    super.initState();
    createEffect(() {
      print('Count: ${_count()}, Doubled: ${_doubled()}');
    });
  }

  @override
  Component build(BuildContext context) {
    return Focusable(
      focused: true,
      onKeyEvent: (event) {
        if (event.logicalKey == LogicalKey.space) {
          _count.value++;
          return true;
        }
        return false;
      },
      child: Center(
        child: Text('Count: ${_count()}, Doubled: ${_doubled()}'),
      ),
    );
  }
}

Test plan

• Run tests with dart test in the signals_nocterm package
• Verify example works with a Nocterm application
• Check that signals are properly disposed when components unmount

Notes

This is a draft PR - happy to iterate on the API based on feedback from @rodydavis and the community.