Keep CubeBackend::put returning () for source compat across cubesys/cubecrypt/
cube-bench; add put_checked() (default Ok(())) letting DaemonBackend report a
real socket error. FUSE create/write/truncate now call put_record_checked and
map the failure to FsError::WriteFailed -> errno 5 (EIO).
Verified: ./check (fmt+test+clippy) green; canonical ./check mount 57/57.
