fix(simulation): use non-blocking TCP to prevent game loop stall

read_framed() called read_exact() which blocked the entire bevy Update
schedule waiting for client input — no systems ran until a keystroke
arrived. Switch TcpStream to non-blocking mode so receive_inputs()
returns Ok(vec![]) on WouldBlock instead of blocking. Toggle to
blocking for snapshot writes (reliable delivery). Add 50ms frame
throttle (~20 ticks/sec) since the non-blocking loop would otherwise
spin. Downgrade input receive logging to trace, add error logging for
missing bridge resource and failed observer queries.

Fixes bug #1 (server never sends snapshots) and #2 (camera doesn't
center until first keystroke).

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

server/src/main.rs

@@ -226,13 +226,23 @@ fn main() {
 
     tracing::info!("Simulation initialized, entering game loop");
 
-    // Game loop: run until client disconnects
+    // Game loop: run until client disconnects.
+    // Targets ~20 ticks/sec (2 game-minutes/sec). The TCP bridge uses
+    // non-blocking reads, so without throttling this loop would spin.
+    // Remaining frame budget is available for NPC AI and pathfinding.
+    let target_frame_time = std::time::Duration::from_millis(50);
     loop {
+        let frame_start = std::time::Instant::now();
+
         app.update();
-        // Check ServerRunning resource
         if !app.world().resource::<ServerRunning>().0 {
             break;
         }
+
+        let elapsed = frame_start.elapsed();
+        if elapsed < target_frame_time {
+            std::thread::sleep(target_frame_time - elapsed);
+        }
     }
 
     tracing::info!("Simulation server shutting down");