PVS pooling

- Pool a lot more stuff with PVS under the assumption once GameState / MsgState are serialized it's safe to fuck with it. I tried using Collections.Pooled although then you need IList and it becomes a lot more scrunkly with serializing it so I just kept using ObjectPool for now. Maybe someday it'd be better to swap as the pooled data could be re-used across more systems although this PVS data allocs to shit on live.
- Move _deletionHistory out of PVSCollection to PvsSystem with the goal of nuking RobustTree someday.

Robust.Server/GameStates/PvsCollection.cs

@@ -37,12 +37,6 @@ public interface IPVSCollection
 
     public void RemoveMap(MapId mapId);
 
-    /// <summary>
-    /// Remove all deletions up to a <see cref="GameTick"/>.
-    /// </summary>
-    /// <param name="tick">The <see cref="GameTick"/> before which all deletions should be removed.</param>
-    public void CullDeletionHistoryUntil(GameTick tick);
-
     public bool IsDirty(IChunkIndexLocation location);
 
     public bool MarkDirty(IChunkIndexLocation location);
@@ -102,11 +96,6 @@ public static Vector2i GetChunkIndices(Vector2 coordinates)
     /// </summary>
     private readonly Dictionary<ICommonSession, HashSet<TIndex>> _lastSeen = new();
 
-    /// <summary>
-    /// History of deletion-tuples, containing the <see cref="GameTick"/> of the deletion, as well as the <see cref="TIndex"/> of the object which was deleted.
-    /// </summary>
-    private readonly List<(GameTick tick, TIndex index)> _deletionHistory = new();
-
     /// <summary>
     /// An index containing the <see cref="IIndexLocation"/>s of all <see cref="TIndex"/>.
     /// </summary>
@@ -151,7 +140,6 @@ public void Process()
 
         foreach (var (index, tick) in _removalBuffer)
         {
-            _deletionHistory.Add((tick, index));
             _changedIndices.Remove(index);
             var location = RemoveIndexInternal(index);
             if (location == null)
@@ -341,51 +329,6 @@ public void RemoveIndex(GameTick tick, TIndex index)
         _removalBuffer[index] = tick;
     }
 
-    /// <inheritdoc />
-    public void CullDeletionHistoryUntil(GameTick tick)
-    {
-        if (tick == GameTick.MaxValue)
-        {
-            _deletionHistory.Clear();
-            return;
-        }
-
-        for (var i = _deletionHistory.Count - 1; i >= 0; i--)
-        {
-            var hist = _deletionHistory[i].tick;
-            if (hist <= tick)
-            {
-                _deletionHistory.RemoveSwap(i);
-                if (_largestCulled < hist)
-                    _largestCulled = hist;
-            }
-        }
-    }
-
-    private GameTick _largestCulled;
-
-    public List<TIndex>? GetDeletedIndices(GameTick fromTick)
-    {
-        if (fromTick == GameTick.Zero)
-            return null;
-
-        // I'm 99% sure this can never happen, but it is hard to test real laggy/lossy networks with many players.
-        if (_largestCulled > fromTick)
-        {
-            _sawmill.Error($"Culled required deletion history! culled: {_largestCulled}. requested: > {fromTick}");
-            _largestCulled = GameTick.Zero;
-        }
-
-        var list = new List<TIndex>();
-        foreach (var (tick, id) in _deletionHistory)
-        {
-            if (tick > fromTick)
-                list.Add(id);
-        }
-
-        return list.Count > 0 ? list : null;
-    }
-
     #endregion
 
     #region UpdateIndex

Robust.Server/GameStates/PvsSystem.Deletion.cs

@@ -0,0 +1,56 @@
+using System.Collections.Generic;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Timing;
+using Robust.Shared.Utility;
+
+namespace Robust.Server.GameStates;
+
+internal sealed partial class PvsSystem
+{
+    /// <summary>
+    /// History of deletion-tuples, containing the <see cref="GameTick"/> of the deletion, as well as the <see cref="TIndex"/> of the object which was deleted.
+    /// </summary>
+    private readonly List<(GameTick tick, NetEntity ent)> _deletionHistory = new();
+
+    /// <inheritdoc />
+    public void CullDeletionHistoryUntil(GameTick tick)
+    {
+        if (tick == GameTick.MaxValue)
+        {
+            _deletionHistory.Clear();
+            return;
+        }
+
+        for (var i = _deletionHistory.Count - 1; i >= 0; i--)
+        {
+            var hist = _deletionHistory[i].tick;
+            if (hist <= tick)
+            {
+                _deletionHistory.RemoveSwap(i);
+                if (_largestCulled < hist)
+                    _largestCulled = hist;
+            }
+        }
+    }
+
+    private GameTick _largestCulled;
+
+    public void GetDeletedEntities(GameTick fromTick, IList<NetEntity> ents)
+    {
+        if (fromTick == GameTick.Zero)
+            return;
+
+        // I'm 99% sure this can never happen, but it is hard to test real laggy/lossy networks with many players.
+        if (_largestCulled > fromTick)
+        {
+            Log.Error($"Culled required deletion history! culled: {_largestCulled}. requested: > {fromTick}");
+            _largestCulled = GameTick.Zero;
+        }
+
+        foreach (var (tick, id) in _deletionHistory)
+        {
+            if (tick > fromTick)
+                ents.Add(id);
+        }
+    }
+}

Robust.Server/GameStates/PvsSystem.Dirty.cs

@@ -18,8 +18,8 @@ internal sealed partial class PvsSystem
         /// <summary>
         /// if it's a new entity we need to GetEntityState from tick 0.
         /// </summary>
-        private HashSet<EntityUid>[] _addEntities = new HashSet<EntityUid>[DirtyBufferSize];
-        private HashSet<EntityUid>[] _dirtyEntities = new HashSet<EntityUid>[DirtyBufferSize];
+        private readonly HashSet<EntityUid>[] _addEntities = new HashSet<EntityUid>[DirtyBufferSize];
+        private readonly HashSet<EntityUid>[] _dirtyEntities = new HashSet<EntityUid>[DirtyBufferSize];
         private int _currentIndex = 1;
 
         private void InitializeDirty()