KsErrorMapping
Describes one @KsError(code, type) binding captured by the ksrpc compiler plugin on a @KsMethod function. The plugin emits one entry per annotation in declaration order into RpcMethod.errorMappings. Runtime routing (#78) consumes these via the forwardErrorMap / reverseErrorMap helpers on RpcMethod — forward maps for client-side deserialization by incoming wire code, reverse maps for server-side resolution of a thrown data::class back to its code + serializer.