Which of a child's two captured streams a decoded line came from. Handed to `ICapturePolicy.OnCapture` so one policy can treat the two streams differently (redact only the stream a passphrase prompt is echoed on, say). It mirrors `OutputEvent`'s stdout/stderr split, but is a bare discriminator carrying no line: the capture seam should not force a consumer that only wants to shape text to depend on the streaming event type. Deliberately carries no stable machine identifier (no `Name`/`FromName`, and no entry in `spec/identifiers.json`), unlike `RlimitResource` or `IoPriorityClass`: nothing serializes it — it is an argument passed to a callback within one process, never a value ProcessKit writes to a report, a log field, or a cassette.