- Repository#append previously surfaced raw PG::TRSerializationFailure when SERIALIZABLE transactions lost the race at COMMIT, forcing callers to rescue both that and AppendConditionViolated to handle what is, semantically, the same outcome: the conditional append did not hold. Map SQLSTATE 40001 alongside P0001 so callers see one exception regardless of which writer won. - ActiveRecordAdapter wraps PG errors in ActiveRecord::StatementInvalid, so the Repository's PG::Error rescue never saw them. Mirror the SequelAdapter pattern and unwrap to the original PG::Error so the mapping applies uniformly across all three adapters. - Narrow the stress-test rescue and the README's optimistic-concurrency example to AppendConditionViolated only, documenting the new contract.