Function session_op_id 

pub fn session_op_id(op: &SessionOp) -> Option<&str>

The session id an op resolves against, or None for the two begin flavors (which mint one). Shared because every pooled/planed dispatcher routes ops by this id — the replicator’s connection-affine shards and the standalone daemon’s reader-backed read-only plane must extract it identically or an op could reach a session engine that never minted its id.