Function remote_segment_transition 

pub const fn remote_segment_transition(
    from: RemoteSegmentLifecycle,
    to: RemoteSegmentLifecycle,
) -> bool

Kafka’s RemoteLogSegmentState.isValidTransition for a known source state: a forward lifecycle edge, or a self transition, which Kafka admits so retries and failover stay idempotent.

A segment with no source state is the host’s add path, which admits only CopyStarted.