fix: Stop treasure chest guardian spawn farming via stack splits (#2568)

### Summary

Players reported an exploit: decipher a treasure map, then run a ClassicUO/Razor organizer agent that pulls the gold out of the chest in small amounts. Each pull spawned more monsters, turning one chest into an unbounded farmable spawn generator.

### Root cause

`TreasureMapChest.OnItemLifted` grants a 10% guardian spawn roll per first-time-lifted item, deduplicated by the instance-keyed `_lifted` set. But a partial lift goes through `Mobile.LiftItemDupe`, which re-adds the stack remainder to the chest as a **brand-new item instance** (engine-side `AddItem`, bypassing the `CheckHold` block on refilling). Every subsequent pull lifts an instance the `_lifted` set has never seen, so each one re-rolls the 10% spawn chance:

- A level 4 chest holds 4,000 gold → pulled coin by coin, ~400 spawned creatures (plus more from reagent stacks), hands-free, per chest.
- Spawns use `guardian: false`, so nothing tracks or caps them.
- Legit full-stack looting yields roughly 5–8 bonus spawns per chest for comparison.

The code is inherited from RunUO, so descendant shards likely share the hole.

### Fix

Mark every item that enters the chest **after the initial fill** as already lifted, via an `OnItemAdded` override gated by a non-serialized `_filled` flag (set at the end of the constructor and in `[AfterDeserialization]`). Ordering makes this exact: `LiftItemDupe` re-adds the remainder *before* the chest's `OnItemLifted` runs, so the lifted original still gets its one legitimate roll while the remainder is pre-marked.

This also covers packing items *into* the chest (e.g., merging gold back in to lift it out again) and bounce-backs — anything not part of the original loot can never grant a spawn roll.

### Tests

- `PartialLift_MarksSplitRemainderAsLifted` — drives the real `Mobile.Lift` path with a 1-coin pull and asserts the split remainder is marked (failed before the fix).
- `ItemAddedAfterFill_IsMarkedLifted` — post-fill additions are marked (failed before the fix).
- `OriginalFillLoot_IsNotMarkedLifted` — original loot keeps spawn-roll eligibility.

Full `UOContent.Tests` suite: 701 passed.

Projects/UOContent.Tests/Tests/Items/TreasureChests/TreasureMapChestLiftTests.cs

@@ -0,0 +1,111 @@
+using Server;
+using Server.Items;
+using Server.Mobiles;
+using Server.Tests;
+using Xunit;
+
+namespace UOContent.Tests;
+
+[Collection("Sequential UOContent Tests")]
+public class TreasureMapChestLiftTests
+{
+    // Coordinates chosen to avoid overlap with Tracking (1000-4000, 1000-4000) and
+    // DetectHidden (1000-2400, 500) test areas.
+
+    [Fact]
+    public void PartialLift_MarksSplitRemainderAsLifted()
+    {
+        using var rng = new PredictableRandom(10); // RandomDouble() = 0.5, no spawn roll fires
+        var map = Map.Felucca;
+        var location = new Point3D(5000, 600, 0);
+        var player = CreatePlayerMobile(map, location);
+        var chest = new TreasureMapChest(1);
+
+        try
+        {
+            chest.MoveToWorld(location, map);
+            chest.Locked = false;
+
+            var gold = FindGold(chest, null);
+            Assert.NotNull(gold);
+
+            player.Lift(gold, 1, out var rejected, out _);
+            Assert.False(rejected);
+
+            // The stack split re-adds the remainder as a brand-new item. It must count as
+            // already lifted, otherwise every 1-coin pull grants a fresh guardian spawn roll.
+            var remainder = FindGold(chest, gold);
+            Assert.NotNull(remainder);
+            Assert.Contains(remainder, chest.Lifted);
+            Assert.Contains(gold, chest.Lifted);
+        }
+        finally
+        {
+            player.Holding?.Delete();
+            player.Delete();
+            chest.Delete();
+        }
+    }
+
+    [Fact]
+    public void ItemAddedAfterFill_IsMarkedLifted()
+    {
+        using var rng = new PredictableRandom(10);
+        var chest = new TreasureMapChest(1);
+        var packed = new Gold(500);
+
+        try
+        {
+            // Anything entering the chest after the initial fill (packed-back gold, split
+            // remainders, GM drops) was never part of the original loot and must not
+            // grant spawn rolls when lifted back out.
+            chest.DropItem(packed);
+
+            Assert.Contains(packed, chest.Lifted);
+        }
+        finally
+        {
+            chest.Delete();
+        }
+    }
+
+    [Fact]
+    public void OriginalFillLoot_IsNotMarkedLifted()
+    {
+        using var rng = new PredictableRandom(10);
+        var chest = new TreasureMapChest(1);
+
+        try
+        {
+            // The original loot must stay roll-eligible for its first lift.
+            Assert.True(chest.Lifted == null || chest.Lifted.Count == 0);
+        }
+        finally
+        {
+            chest.Delete();
+        }
+    }
+
+    private static Gold FindGold(TreasureMapChest chest, Gold except)
+    {
+        var items = chest.Items;
+
+        for (var i = 0; i < items.Count; i++)
+        {
+            if (items[i] is Gold gold && gold != except)
+            {
+                return gold;
+            }
+        }
+
+        return null;
+    }
+
+    private static PlayerMobile CreatePlayerMobile(Map map, Point3D location)
+    {
+        var mobile = new PlayerMobile(World.NewMobile);
+        mobile.DefaultMobileInit();
+        mobile.MoveToWorld(location, map);
+        return mobile;
+    }
+}