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/lib.rs

@@ -2,8 +2,10 @@
 
 pub mod config;
 pub mod error;
+pub mod kdl_parse;
 pub mod state;
 
 pub use config::{RestartConfig, RestartPolicy, ServerConfig, StopConfig};
 pub use error::{ConfigError, RpcErrorCode};
+pub use kdl_parse::parse_server_config;
 pub use state::ServerState;