No store is primary. An entity may live in a single store or be spread
across all four. Identity, transactions, time and migrations never assume a
master database.
Native query languages stay. SQL, Cypher and Iceberg scans are not
unified into a DSL; the layer unifies identity, transactions, time and
result access.
ACID belongs to the layer, not only to the stores, and is stated
honestly where a store limits it (see Guarantees).
Portable by design. No Python-only mechanisms in the core contracts
(identity, journal, protocol, migration format).