Surface that external code uses to extend ReifyDB - building operators, procedures, transforms, flows, and custom connectors that plug into the engine. The crate exposes the FFI boundary that out-of-process extensions implement, the marshalling for values that cross that boundary, and a testing harness so an extension author can run their code against an in-process engine without a server.
The shapes of catalog objects, RQL fragments, and result rows that an extension sees here are stable contracts: the SDK is what insulates third-party code from internal refactors of the engine, the planner, or the storage tier. Anything that crosses this boundary - identifiers, errors, columnar payloads - has a versioned representation, and widening that representation requires a coordinated bump on both sides.
Invariant: the FFI layer does not leak engine-internal types; everything an extension sees comes either from
reifydb-type or from this crate's own re-exports. Reaching for an internal engine type from inside an SDK module
ties extension ABI to engine refactors.