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/error.rs

@@ -4,7 +4,11 @@ use thiserror::Error;
 #[derive(Debug, Error)]
 pub enum ConfigError {
     #[error("failed to read {path}: {source}")]
-    Io { path: PathBuf, #[source] source: std::io::Error },
+    Io {
+        path: PathBuf,
+        #[source]
+        source: std::io::Error,
+    },
 
     #[error("failed to parse KDL in {path}: {message}")]
     Parse { path: PathBuf, message: String },
@@ -13,10 +17,18 @@ pub enum ConfigError {
     MissingField { path: PathBuf, field: &'static str },
 
     #[error("invalid value for `{field}` in {path}: {message}")]
-    InvalidValue { path: PathBuf, field: &'static str, message: String },
+    InvalidValue {
+        path: PathBuf,
+        field: &'static str,
+        message: String,
+    },
 
     #[error("duplicate port {port} declared by both `{name_a}` and `{name_b}`")]
-    DuplicatePort { name_a: String, name_b: String, port: u16 },
+    DuplicatePort {
+        name_a: String,
+        name_b: String,
+        port: u16,
+    },
 
     #[error("server name `{name}` contains invalid characters (allowed: a-z, 0-9, '-', '_')")]
     InvalidName { name: String },
@@ -33,5 +45,7 @@ pub enum RpcErrorCode {
 }
 
 impl RpcErrorCode {
-    pub fn as_i32(self) -> i32 { self as i32 }
+    pub fn as_i32(self) -> i32 {
+        self as i32
+    }
 }