Refactor KVEngine and Storage engine to implement same trait

- KVEngine is added to io module however base engine lives in storage and provides the engine primitives.
- Ensure KVEngine implements engine similar to storage engine

Start by reading the KVEngine definition in the io module and the base engine implementation in storage, then compare their engine primitives and trait boundaries. Done means both engines implement the same trait while preserving the existing storage primitives; run the repository's existing tests to check for regressions.

Written by the indexing model from the issue text.