Real-llama.cpp backend for erllama_model.

Owns a model_ref and a context_ref from erllama_nif. The gen_statem hands its decode/kv operations through this module; this module forwards to the NIF.

Config (passed through erllama_model:start_link/2): model_path :: file:name() | binary() (required) model_opts :: map() (forwarded to erllama_nif:load_model/2) context_opts :: map() (forwarded to erllama_nif:new_context/2)