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/Engines/Help/StuckMenu.cs

@@ -300,7 +300,7 @@ namespace Server.Menus.Questions
                     }
                     else
                     {
-                        destMap = m_Mobile.Kills >= 5 ? Map.Felucca : Map.Trammel;
+                        destMap = m_Mobile.Murderer ? Map.Felucca : Map.Trammel;
                     }
 
                     BaseCreature.TeleportPets(m_Mobile, dest, destMap);