Change MessageType from u16 to u32

Breaking wire protocol change: MessageType field increased from 2 bytes
to 4 bytes in LP packets. This future-proofs the message type space and
aligns with other u32 fields.

Changes:
- message.rs: #[repr(u32)], from_u32(), to_u32()
- error.rs: InvalidMessageType(u32)
- codec.rs: All serialization/deserialization updated to 4-byte msg_type
  - Cleartext parsing: inner_bytes[4..8], content at [8..]
  - AEAD parsing: decrypted[4..8], content at [8..]
  - Serialization: 4 bytes for message type

gateway/src/node/lp_listener/handler.rs

@@ -453,7 +453,12 @@ impl LpConnectionHandler {
         // Get outer key before releasing borrow
         let outer_key = state_machine
             .session()
-            .map_err(|e| GatewayError::LpProtocolError(format!("Session unavailable after processing: {}", e)))?
+            .map_err(|e| {
+                GatewayError::LpProtocolError(format!(
+                    "Session unavailable after processing: {}",
+                    e
+                ))
+            })?
             .outer_aead_key();
         drop(state_entry);
 
@@ -1130,8 +1135,9 @@ mod tests {
             timestamp_tolerance_secs: 30,
             ..Default::default()
         };
-        let forward_semaphore =
-            Arc::new(tokio::sync::Semaphore::new(lp_config.max_concurrent_forwards));
+        let forward_semaphore = Arc::new(tokio::sync::Semaphore::new(
+            lp_config.max_concurrent_forwards,
+        ));
 
         LpHandlerState {
             lp_config,

gateway/src/node/lp_listener/mod.rs

@@ -361,7 +361,7 @@ pub struct LpHandlerState {
     /// Prevents file descriptor exhaustion when forwarding LP packets during
     /// telescope setup. When at capacity, forward requests return an error
     /// so clients can choose a different gateway.
-    // AIDEV-NOTE: Connection limiting (not pooling) chosen for forward requests.
+    // Connection limiting (not pooling) chosen for forward requests.
     //
     // Why not connection pooling?
     // 1. Forwarding is one-time per telescope setup (handshake only), not ongoing traffic.