Prepared query form — opaque payload held by the caller between
prepare_query and distance_prepared calls. New variants will be added
as specific codec impls land in later sub-tasks.
Object-safe trait for asymmetric rerank codecs. Each impl wraps an existing
nodedb-codec::VectorCodec and exposes a uniform shape so the sidecar can
hold Arc<dyn RerankCodec> regardless of the underlying associated-type
machinery.