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/Migrations/Server.Items.TreasureMapChest.v3.json

@@ -0,0 +1,57 @@
+{
+  "version": 3,
+  "type": "Server.Items.TreasureMapChest",
+  "properties": [
+    {
+      "name": "Guardians",
+      "type": "System.Collections.Generic.List\u003CServer.Mobile\u003E",
+      "rule": "ListMigrationRule",
+      "ruleArguments": [
+        "@Tidy",
+        "@CanBeNull",
+        "Server.Mobile",
+        "SerializableInterfaceMigrationRule"
+      ]
+    },
+    {
+      "name": "Temporary",
+      "type": "bool",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        ""
+      ]
+    },
+    {
+      "name": "Owner",
+      "type": "Server.Mobile",
+      "rule": "SerializableInterfaceMigrationRule"
+    },
+    {
+      "name": "Level",
+      "type": "int",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        ""
+      ]
+    },
+    {
+      "name": "ExpireTimer",
+      "type": "Server.Timer",
+      "rule": "TimerMigrationRule",
+      "ruleArguments": [
+        "@TimerDrift"
+      ]
+    },
+    {
+      "name": "Lifted",
+      "type": "System.Collections.Generic.HashSet\u003CServer.Item\u003E",
+      "rule": "HashSetMigrationRule",
+      "ruleArguments": [
+        "@Tidy",
+        "@CanBeNull",
+        "Server.Item",
+        "SerializableInterfaceMigrationRule"
+      ]
+    }
+  ]
+}