feat: make llama.cpp a deferred backend on macOS/Windows

[doringeman]

MODEL_RUNNER_PORT=8080 make run LLAMA_SERVER_VERSION=v0.1.0

docker/model-runner:llamacpp-metal-v0.1.0

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model run smollm2 hi
Installing llama.cpp backend...
llama.cpp backend installed successfully
Hello. I'm happy to help you with any questions or queries you may have. What would you like to discuss today?

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model status
Docker Model Runner is running

BACKEND    STATUS         DETAILS
llama.cpp  Running        llama.cpp llamacpp-metal-v0.1.0 (sha256:6a242e1021bc7dc504efcd9c59dca998aa5e9cf7148b9dfa514d1dae4582b952) 3191462
vllm       Running        vllm-metal v0.1.0-20260126-121650
diffusers  Not Installed
mlx        Not Installed  package not installed
sglang     Not Installed  only supported on Linux

[gemini-code-assist]

Code Review

This pull request refactors the llama.cpp backend to support deferred installation on macOS and Windows, which is a great improvement. The changes are well-structured, introducing a unified installDir and a BackendUpdater interface for managing binary updates. My main concern is with error handling in main.go where a failure to determine the llama.cpp server path is logged but not treated as a fatal error, which could lead to issues later on. I've added a critical comment with a suggestion to address this.

[gemini-code-assist]

The program continues to run even if envconfig.LlamaServerPath() returns an error. If this happens, llamaServerPath will be an empty string, which will cause the model runner to attempt to write to the current working directory instead of the intended installation directory. This can lead to unexpected behavior and potential permission issues. This error should be treated as fatal, and the program should exit.

Suggested change

	if err != nil {
	log.Error("Failed to get llama.cpp server path", "error", err)
	}
	if err != nil {
	log.Error("Failed to get llama.cpp server path", "error", err)
	exitFunc(1)
	}