feat: Adds Mobile.Murderer virtual property, consolidates kill-threshold checks (#2355)

## Summary

- Adds `public virtual bool Murderer => Kills >= 5` property to `Mobile`, replacing ~30 scattered `Kills >= 5` / `Kills < 5` magic-number checks across the codebase
- `BaseCreature` overrides `Murderer` to also return `true` when `AlwaysMurderer` is set
- Updates `Corpse` serialization to v15, storing `_murderer` as a `bool` field (migrated from `int Kills >= 5` in earlier versions)

Projects/UOContent/Items/Misc/Moonstone.cs

@@ -64,7 +64,7 @@ public partial class Moonstone : Item
         {
             mobile.SendLocalizedMessage(1049543); // You decide against traveling to Felucca while you are still young.
         }
-        else if (from.Kills >= 5)
+        else if (from.Murderer)
         {
             // The magic of the stone cannot be evoked by someone with blood on their hands.
             from.SendLocalizedMessage(1005402);