Function transaction_completion_decision 

pub fn transaction_completion_decision(
    current: TransactionSnapshot,
    expected: TransactionIdentity,
    completion: TransactionIdentity,
    prepare_state: i8,
    complete_state: i8,
) -> TransactionCompletionDecision

Revalidate the transaction entry after the marker fan-out released its lock.

expected and prepare_state are what the caller prepared; completion and complete_state are the completion it intends to write. The live entry already holding that completion is an idempotent success; otherwise the entry must still hold the prepared identity (else it was fenced) and the prepared state (else another caller moved it).

The two state tags are one Kafka Prepare* -> Complete* pairing, which the host takes from a fixed mapping, so they differ.