Function upsert_source_offset 

pub fn upsert_source_offset(
    txn: &mut ApplyTxn,
    source: &str,
    offset: &str,
    chunk_seq: i64,
    run_id: Option<&str>,
) -> Result<(), ReplicaError>

Upsert the durable (offset, chunk_seq) checkpoint for source into txn (co-transactional with the effects that txn carries — a crash can never land data without its cursor, §4). Shared by the whole-batch (ApplyConsumer::commit_normalized_with_offset) and streaming-follower (ApplyConsumer::commit_follower_txn) commit paths. A whole-run commit passes [SOURCE_OFFSET_WHOLE_RUN]; a mid-run segment passes the boundary chunk’s chunk_seq. run_id is the run’s identity token from its begin frame (the fencing proof echoed on the next subscribe — RELAY-CURSOR-EPOCH-FENCING-DESIGN.md §2); None (a pre-fence upstream or a non-streaming source) stores NULL, which subscribes without a fence as before.