fix(bridge): add Disconnected error variant, fix test race condition

Replace string-matching disconnect detection with explicit
BridgeError::Disconnected variant. Add TcpBridge::accept_on(listener)
that takes a pre-bound TcpListener, eliminating the 100ms sleep hack
in TCP tests. Send errors now also trigger ServerRunning=false.
Add trace logging to generate_snapshot for entity count visibility.

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

server/src/bridge/tcp.rs

@@ -56,6 +56,36 @@ impl TcpBridge {
         })
     }
 
+    /// Server-side: accept one connection on an existing TcpListener.
+    /// Avoids race conditions in tests by separating bind from accept.
+    pub fn accept_on(listener: TcpListener) -> Result<Self, BridgeError> {
+        let local_addr = listener
+            .local_addr()
+            .map_err(|e| BridgeError::Transport(format!("failed to get local address: {}", e)))?;
+
+        tracing::info!("TcpBridge accepting on {}", local_addr);
+
+        let (stream, peer_addr) = listener
+            .accept()
+            .map_err(|e| BridgeError::Transport(format!("failed to accept connection: {}", e)))?;
+
+        tracing::info!(
+            "TcpBridge accepted connection from {} on {}",
+            peer_addr,
+            local_addr
+        );
+
+        let reader_stream = stream.try_clone().map_err(|e| {
+            BridgeError::Transport(format!("failed to clone stream for reader: {}", e))
+        })?;
+
+        Ok(Self {
+            reader: Mutex::new(BufReader::new(reader_stream)),
+            writer: Mutex::new(BufWriter::new(stream)),
+            local_addr,
+        })
+    }
+
     /// Client-side: connect to TCP address (for tests).
     pub fn connect(addr: &str) -> Result<Self, BridgeError> {
         tracing::info!("TcpBridge connecting to {}", addr);
@@ -108,7 +138,7 @@ impl SimBridge for TcpBridge {
                 tracing::trace!("received {} inputs", inputs.len());
                 Ok(inputs)
             }
-            None => Err(BridgeError::Transport("client disconnected (EOF)".into())),
+            None => Err(BridgeError::Disconnected),
         }
     }
 }