Function internode_rpc_msgpack_only 

pub fn internode_rpc_msgpack_only() -> bool

Whether internode metadata RPCs should send only the msgpack _bin payloads and leave the JSON compatibility strings empty (grpc-optimization P2-1). Shared by the client (remote_disk) and server (node_service) send paths.

The legacy flag alone is deliberately insufficient: emptying JSON breaks old peers that only decode the compatibility field. Operators must also set the fleet-confirmed guard after the convergence runbook proves every peer supports _bin and rollback.