Party Loot and Drop System Update

This update improves party-based loot distribution, drop-rate calculations, and several related systems.

### Individual Party Loot Rolls

The behavior of `<SELECT>` loot groups has been updated.

Previously, one item was selected for the entire party based on a single player’s luck. Each party member now receives an individual roll based on their own drop chance.

As a result:

- Drop-rate buffs are applied separately to each player.

- Party members may receive different items from the same `<SELECT>` group.

- Each result is determined by the individual player’s luck.

A calculation issue that prevented certain items in `<SELECT>` groups from dropping has also been fixed.

Drop-rate buffs are now applied more consistently and provide their intended effect.

### System Improvements

- Added a global loot system that allows special items to drop from any monster during events or under special conditions.

- Refactored the `DropByKill` function to improve modularity and code reuse.

- Fixed an issue that could remove every party member from a dungeon when one player left under specific conditions.

- Added an automatic loot function designed for events involving large groups of monsters.

- Fixed last-hit logic so that loot assigned to the final attacker is awarded correctly.