feat(halo): add llama-server systemd unit for Qwen3.6-35B-A3B

Runs llama.cpp's ROCm build under DynamicUser, with the HF model cache
in StateDirectory (survives systemctl clean) and KV slot saves in
CacheDirectory. Listens on :8000.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

systems/x86_64-linux/halo/default.nix

@@ -10,6 +10,7 @@ with lib.metacfg;
     ./hardware-configuration.nix
     #./xremap.nix
     ./wyoming.nix
+    ./llama-server.nix
   ];
 
   boot.lanzaboote.pkiBundle = "/var/lib/sbctl";