fix(bridge): use named MessagePack format for wire compatibility

Client-side Protocol.gd expects rmp_serde::to_vec_named() (maps with
string keys), but LocalBridge was using to_vec() (compact positional
arrays). Fix send_snapshot and update all test serialization calls to
match actual wire format. Also change EOF from Ok(vec![]) to
BridgeError::Transport so bridge systems can detect disconnects.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/bridge/local.rs

@@ -81,7 +81,7 @@ impl LocalBridge {
 
 impl SimBridge for LocalBridge {
     fn send_snapshot(&self, snapshot: &ObserverSnapshot) -> Result<(), BridgeError> {
-        let payload = rmp_serde::to_vec(snapshot)?;
+        let payload = rmp_serde::to_vec_named(snapshot)?;
 
         let mut writer = self.writer.lock().expect("writer mutex poisoned");
         write_framed(writer.get_mut(), &payload)?;
@@ -99,10 +99,7 @@ impl SimBridge for LocalBridge {
                 tracing::trace!("received {} inputs", inputs.len());
                 Ok(inputs)
             }
-            None => {
-                tracing::trace!("received EOF, returning empty input vec");
-                Ok(Vec::new())
-            }
+            None => Err(BridgeError::Transport("client disconnected (EOF)".into())),
         }
     }
 }