NIXL data plane

The NIXL path is optional. It needs libnixl and UCX on Linux, using either RDMA NICs or shared-memory transports. On a development machine without that runtime, typecheck the code path against stubs:

cargo check --features nixl-stub

On Linux with NIXL installed, split a prefill and a decode engine:

# prefill
cargo run --features nixl -- play --pd-role prefill \
  --engine-id mock-prefill --side-channel-host 127.0.0.1 --side-channel-port 5600 ...

# decode
cargo run --features nixl -- play --pd-role decode \
  --engine-id mock-decode --side-channel-port 5601 ...

The transfer path uses remote_host and remote_port from kv_transfer_params to fetch the prefill's PoolDescriptor over a TCP metadata side channel, then issues NIXL READs for the advertised block ids. Decode receives those remote_* fields per request; the prefill address is not a decode CLI argument.

tests/nixl_loopback.rs validates the byte-transfer path with distinct prefill and decode agents in one process. Kubernetes deployment validation is separate.