ExoChain DAG DB API DTOs.
exo-dag-db-api owns these wire shapes. exo-api::dagdb re-exports them
as the stable compatibility path for gateway, node, SDK, and external
callers.
Every consumer-facing REST response DTO carries a stable
schema_version string constant so a non-Rust integrator can detect the
wire-contract version directly from a response body. The constants below are
the single source of truth; the checked-in machine contract under
docs/dagdb/api/ and the fixture round-trip tests are asserted against them.