Recursion bug due to module.build() connection tracing

[desmonddak]

Describe the bug

With some module miswirings we can get a StackOverflow due to infinite recursion.

To Reproduce

Run the following test:

import 'package:rohd/rohd.dart';
import 'package:test/test.dart';

class EmptyModule extends Module {
  EmptyModule({super.name});
}

class RecursivePortCheckRepro extends Module {
  RecursivePortCheckRepro() : super(name: 'top') {
    final driver = Logic();
    final topA = addInput('a', driver);

    final child = EmptyModule(name: 'child');
    final childA = child.addInput('a', topA);

    // The `and()` avoids Logic's immediate direct self-connection rejection
    // while closing the signal graph.
    driver <= childA.and();
  }
}

void main() {
  test('discovers a cyclic hierarchy', () async {
    await expectLater(
      RecursivePortCheckRepro().build(),
      throwsA(isA<InvalidHierarchyException>()),
    );
  });
}

Expected behavior

An error reported on the bad connection in the parent to a child Logic.

Actual behavior

StackOverflow

Additional: Dart SDK info

No response

Additional: pubspec.yaml

Additional: Context

The simplest fix would look like:

void _checkPortConnectionsRecursively({
  Set<Module>? visited,
}) {
  final marked = visited ?? <Module>{};

  // Mark before descending. This breaks cycles such as top -> child -> top.
  if (!marked.add(this)) {
    return;
  }

  _checkPortConnections();

  for (final subModule in _subModules) {
    subModule._checkPortConnectionsRecursively(visited: marked);
  }
}