Implement control channel routing for wraith-* reserved destinations (ADR-018)

- Add control_channel.rs with WRAITH_CONTROL_DESTINATION, WRAITH_PREFIX constants
- Add ControlChannelHandler trait and ControlChannelRouter for routing logic
- Add DuplexStream supertrait for Box<dyn> compatibility
- Server handler rejects wraith-* destinations when no handler configured
- Add ForwardError type to fix pre-existing compilation error
- Unit tests: reserved detection, non-reserved pass-through, prefix matching

crates/wraith-core/src/error.rs

@@ -60,6 +60,22 @@ pub enum ConfigError {
     IncompatibleOptions,
 }
 
+#[derive(Debug, thiserror::Error)]
+pub enum ForwardError {
+    #[error("invalid forward specification: {spec}")]
+    InvalidSpec { spec: String },
+    #[error("bind failed")]
+    BindFailed {
+        #[source]
+        source: io::Error,
+    },
+    #[error("channel open failed")]
+    ChannelOpenFailed {
+        #[source]
+        source: Box<dyn std::error::Error + Send + Sync>,
+    },
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;