Apply and reverse a versioned edit

Implement apply_transaction(state, expected_version, changes) and rollback(state, receipt). State has an integer version and a fields dictionary of immutable scalar values. Applying requires a matching version and known fields, returns a new state at version+1 plus a receipt containing a copy of prior fields and after_version. Even an empty changes dictionary is a committed transaction. Rollback requires the state's version to equal receipt.after_version; it restores prior fields in a new state and increments the version again. Never mutate inputs. Reject stale versions and unknown fields with ValueError.