SynthIssue class sealed

One reason a Stack cannot be synthesized.

Stack.synth() and Stack.writeTo() check the whole Stack first and throw one SynthException listing every issue; Stack.validate() returns them without throwing. Each subtype carries the facts a test can match on:

for (final issue in stack.validate()) {
  switch (issue) {
    case UnregisteredReference(:final address, :final target):
      print('$address reads $target, which was never added');
    default:
      print(issue);
  }
}