Switch the C wrapper from the lower-level registerReqFFI + hand-written
exports to nim-ffi's declareLibrary/{.ffiCtor.}/{.ffi.}/{.ffiDtor.}
macros. This removes the per-op boilerplate (request structs, shared-mem
copy helpers, dispatch template) in favour of the framework's JSON
marshalling, and lets participantId be threaded in via config.
Consequences (intentional): C exports are now snake_case and parameters
travel as JSON, so the ABI changes and sds-go-bindings must be updated to
match. Two cases can't ride the JSON path: the retrieval-hint provider is
a C function pointer (kept hand-written, dispatched to set a worker-thread
threadvar) and the unwrap response has nested objects the framework's
serializer can't emit (returned as a hand-built JSON string).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
