fix: default DistributedBackend to None_ (absent means single-node)

Also add explicit distributed_backend = "mp" to glm51 recipe
which relies on multi-node distributed execution.

recipes/glm51/recipe.toml

@@ -3,8 +3,9 @@ name = "GLM-5.1 NVFP8"
 docker_image = "vllm/vllm-openai:glm51-cu130"
 model = "lukealonso/GLM-5.1-NVFP4"
 
-required_nodes = 4                  # 0 = любое, иначе строгое совпадение
-pipeline_parallel_size = 0          # 0 = авто (равно nnodes), иначе кастомное значение
+required_nodes = 4
+distributed_backend = "mp"
+pipeline_parallel_size = 0
 
 args = [
     "--speculative-config.method", "mtp",

src/config.rs

@@ -308,7 +308,7 @@ mod tests {
         assert_eq!(recipe.port, 30000);
         assert!(recipe.args.is_empty());
         assert!(recipe.env.is_empty());
-        assert_eq!(recipe.distributed_backend, DistributedBackend::Mp);
+        assert_eq!(recipe.distributed_backend, DistributedBackend::None_);
         assert_eq!(recipe.tensor_parallel_size, 1);
     }
     #[test]
@@ -480,9 +480,9 @@ ib_disable = 0
         assert_eq!(recipe.engine, "vllm");
     }
     #[test]
-    fn test_default_distributed_backend_is_mp() {
+    fn test_default_distributed_backend_is_none() {
         let recipe: Recipe = toml::from_str("name = \"x\"\nmodel = \"m\"").unwrap();
-        assert_eq!(recipe.distributed_backend, DistributedBackend::Mp);
+        assert_eq!(recipe.distributed_backend, DistributedBackend::None_);
     }
     #[test]
     fn default_config_dir_appends_sm121_launcher() {

src/pre_serve.rs

@@ -55,14 +55,14 @@ pub struct ModsTable {
     pub commands: Vec<PreServeCommand>,
 }
 
-/// Distributed execution backend: `mp` (default), `ray`, or `none`.
+/// Distributed execution backend — `mp`, `ray`, or absent (single-node).
+/// Defaults to [`None_`] when not specified in the recipe.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Deserialize)]
 #[serde(rename_all = "lowercase")]
 pub enum DistributedBackend {
-    #[default]
     Mp,
     Ray,
-    #[serde(rename = "none")]
+    #[default]
     None_,
 }