feat(protocol): KDL parser for ServerConfig

Adds kdl_parse module with parse_server_config() that deserialises a
KDL document into ServerConfig, with full validation of name, types,
durations, and restart/stop blocks. Also derives Default on
RestartPolicy to satisfy clippy.

crates/xy-protocol/src/config.rs

@@ -1,20 +1,15 @@
 use serde::{Deserialize, Serialize};
 use std::time::Duration;
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "kebab-case")]
 pub enum RestartPolicy {
     Always,
+    #[default]
     OnFailure,
     Never,
 }
 
-impl Default for RestartPolicy {
-    fn default() -> Self {
-        RestartPolicy::OnFailure
-    }
-}
-
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct RestartConfig {
     #[serde(default)]