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/local.rs

@@ -99,7 +99,7 @@ impl SimBridge for LocalBridge {
                 tracing::trace!("received {} inputs", inputs.len());
                 Ok(inputs)
             }
-            None => Err(BridgeError::Transport("client disconnected (EOF)".into())),
+            None => Err(BridgeError::Disconnected),
         }
     }
 }