fix: Refugee center back room monster group

[Zlorthishen]

Purpose of change

currently, the monsters which spawn in in the refugee center are from the group GROUP_MALL. this is not normally a problem, until multiple mod interactions cause unintended and extra monsters to spawn for this quest.

Describe the solution

creates a new monster group, GROUP_REFUGEE_CENTER_BACK_ROOM which controls which monsters will spawn

Describe alternatives you've considered

Testing

👍

Additional context

Checklist

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[chaosvolt]

No later-game necros and masters like GROUP_MALL has? Fair I suppose since a master can royally screw up the center if it evolves a lot of stuff, hmm.

[Zlorthishen]

Fair I suppose since a master can royally screw up the center if it evolves a lot of stuff, hmm.

yeah, i had that happen before when i had a Next Summer scenario with accelerated monster evolution. half the npcs were dead before i even stole all the gas from the busses

[chaosvolt]

I cri but