The abstract method had no return annotation, so pyrefly inferred -> None
and flagged both DirectoryExportSink.stream and ZipExportSink.stream as
incompatible overrides.
The implementation plan for Task 2 diverged from the design spec
(export-sink-architecture-design.md), leaving ExportSink as a plain
class with NotImplementedError bodies instead of the specified
AbstractContextManager subclass. Use abc.ABC + @abstractmethod so a
concrete sink missing a required method fails at instantiation
rather than at first call.
stumpylog