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/mod.rs

@@ -23,6 +23,8 @@ pub enum BridgeError {
     Io(#[from] std::io::Error),
     #[error("transport error: {0}")]
     Transport(String),
+    #[error("client disconnected")]
+    Disconnected,
 }
 
 /// Abstracts transport layer (D-020)
@@ -86,6 +88,11 @@ pub fn generate_snapshot(
             kind,
         });
     }
+    tracing::trace!(
+        "generate_snapshot: tick={}, entities={}",
+        time.tick,
+        visible.len()
+    );
     buffer.snapshot = Some(ObserverSnapshot {
         tick: time.tick,
         entities: visible,
@@ -105,7 +112,7 @@ pub fn receive_bridge_inputs(
                 input_queue.push(input);
             }
         }
-        Err(BridgeError::Transport(ref msg)) if msg.contains("disconnected") => {
+        Err(BridgeError::Disconnected) => {
             tracing::info!("Client disconnected, shutting down");
             running.0 = false;
         }
@@ -119,11 +126,13 @@ pub fn receive_bridge_inputs(
 pub fn send_bridge_snapshot(
     bridge: Option<Res<BridgeResource>>,
     mut buffer: ResMut<SnapshotBuffer>,
+    mut running: ResMut<ServerRunning>,
 ) {
     let Some(bridge) = bridge else { return };
     if let Some(snapshot) = buffer.snapshot.take() {
         if let Err(e) = bridge.send_snapshot(&snapshot) {
             tracing::error!("Bridge send error: {}", e);
+            running.0 = false;
         }
     }
 }
@@ -150,7 +159,9 @@ impl Plugin for BridgePlugin {
                 Update,
                 (
                     receive_bridge_inputs.before(crate::simulation::input::process_player_input),
-                    generate_snapshot.after(crate::simulation::movement::validate_movement),
+                    generate_snapshot
+                        .after(crate::simulation::movement::validate_movement)
+                        .before(crate::simulation::time::advance_tick),
                     send_bridge_snapshot.after(generate_snapshot),
                 ),
             );