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/ConPVP/TournamentPyramid.cs

@@ -42,7 +42,7 @@ namespace Server.Engines.ConPVP
                             {
                                 var mob = players[j];
 
-                                if (mob.Kills >= 5)
+                                if (mob.Murderer)
                                 {
                                     parts[0].Players.Add(mob);
                                 }