Improve thread safety and performance in core systems

Refactors event, entity, and physics management to use thread-safe and lock-free data structures (Immutable collections, ConcurrentQueue, ConcurrentDictionary, Channel) for improved concurrency and performance. Replaces O(n) queue lookups with O(1) set/dictionary checks, ensures atomic updates for shared state, and optimizes queue draining and deferred action processing. Updates related code to use new APIs and patterns, and adds documentation for thread safety and workflow.

Barotrauma/BarotraumaClient/ClientSource/Networking/EntitySpawner.cs

@@ -1,12 +1,32 @@
 using Barotrauma.Items.Components;
 using Barotrauma.Networking;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 
 namespace Barotrauma
 {
     partial class EntitySpawner : Entity, IServerSerializable
     {
-        public readonly List<(Entity entity, bool isRemoval)> receivedEvents = new List<(Entity entity, bool isRemoval)>();
+        /// <summary>
+        /// Thread-safe queue for received entity spawn/remove events from the server.
+        /// </summary>
+        private readonly ConcurrentQueue<(Entity entity, bool isRemoval)> receivedEventsQueue = new ConcurrentQueue<(Entity entity, bool isRemoval)>();
+
+        /// <summary>
+        /// Gets a thread-safe snapshot of received events.
+        /// </summary>
+        public IEnumerable<(Entity entity, bool isRemoval)> GetReceivedEventsSnapshot()
+        {
+            return receivedEventsQueue.ToArray();
+        }
+
+        /// <summary>
+        /// Clears all received events from the queue.
+        /// </summary>
+        partial void ResetReceivedEvents()
+        {
+            while (receivedEventsQueue.TryDequeue(out _)) { }
+        }
 
         public void ClientEventRead(IReadMessage message, float sendingTime)
         {
@@ -34,7 +54,7 @@ namespace Barotrauma
                 {
                     DebugConsole.Log("Received entity removal message for ID " + entityId + ". Entity with a matching ID not found.");
                 }
-                receivedEvents.Add((entity, true));
+                receivedEventsQueue.Enqueue((entity, true));
             }
             else
             {
@@ -57,7 +77,7 @@ namespace Barotrauma
                                     GameAnalyticsManager.AddDesignEvent("ItemFabricated:" + (GameMain.GameSession?.GameMode?.Preset.Identifier ?? "none".ToIdentifier()) + ":" + newItem.Prefab.Identifier);
                                 }
                             }
-                            receivedEvents.Add((newItem, false));
+                            receivedEventsQueue.Enqueue((newItem, false));
                         }
                         break;
                     case (byte)SpawnableType.Character:
@@ -68,7 +88,7 @@ namespace Barotrauma
                         }
                         else
                         {
-                            receivedEvents.Add((character, false));
+                            receivedEventsQueue.Enqueue((character, false));
                         }
                         break;
                     default: