Relational overlay for DonaDB.
donadb-rel adds typed schemas, secondary indexes, predicate scans, and
foreign-key traversal on top of DonaDB without changing the storage engine.
Records are encoded into opaque DonaDB values, while indexes live in dedicated
domains and are updated in the same write batch as the primary record.
The layer is intentionally additive: raw DonaDB reads and writes can coexist with schema-aware access for applications that need both direct key-value storage and relational query ergonomics.