Apply repeated events exactly once locally

Reduce events with source, id, account, and signed integer delta. Identity is the pair (source,id). Apply the first occurrence, ignore identical repeats, and reject a repeated identity with a different account or delta. Return account balances; negative balances are allowed because these are general counters, not financial accounts. Validate nonempty string identities and integer deltas excluding booleans. This is an in-memory projection exercise, not a claim of end-to-end exactly-once delivery.