Initial implementation: relay server + common protocol + client stub

crates/common/src/error.rs

@@ -0,0 +1,35 @@
+use std::fmt;
+
+#[derive(Debug)]
+pub enum HeliosError {
+    /// WebSocket protocol error
+    Protocol(String),
+    /// JSON serialization/deserialization error
+    Serialization(String),
+    /// Session not found
+    SessionNotFound(String),
+    /// Request timed out waiting for client response
+    Timeout(String),
+    /// Generic internal error
+    Internal(String),
+}
+
+impl fmt::Display for HeliosError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            HeliosError::Protocol(msg) => write!(f, "Protocol error: {msg}"),
+            HeliosError::Serialization(msg) => write!(f, "Serialization error: {msg}"),
+            HeliosError::SessionNotFound(id) => write!(f, "Session not found: {id}"),
+            HeliosError::Timeout(msg) => write!(f, "Request timed out: {msg}"),
+            HeliosError::Internal(msg) => write!(f, "Internal error: {msg}"),
+        }
+    }
+}
+
+impl std::error::Error for HeliosError {}
+
+impl From<serde_json::Error> for HeliosError {
+    fn from(e: serde_json::Error) -> Self {
+        HeliosError::Serialization(e.to_string())
+    }
+}