Consistently use RAFT resources internally

[csadorf]

Summary

After #102, the public API uses raft::device_resources, but nvForest internals still use raft_proto::cuda_stream and helper shims like raft_proto::get_next_usable_stream.

We should follow up and make resource/stream handling consistent throughout the codebase.

Scope

• Keep the public API introduced in Simplify C++ inference with auto-instantiated RAFT handle #102.
• Replace remaining raft_proto resource/stream shims where practical.
• Make sure NVFOREST_ENABLE_GPU=OFF still builds.
• Update Cython bindings, tests, and docs as needed.

Acceptance criteria

• Internal resource/stream usage is consistent.
• CPU-only CI still passes.
• GPU tests still pass.
• Any remaining raft_proto stream/resource helpers are intentional and documented.