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Design a smart locker assignment system with locker sizes: `Small`, `Medium`, `Large`.

Rules:
- Each package has a size requirement
- One package per locker
- Package can go to same size or larger locker
- Always choose the smallest available locker that fits

Example inventory:
- Small: 2
- Medium: 1
- Large: 1

Requests:
- A -> Small
- B -> Small
- C -> Medium
- D -> Small

Expected allocation:
- A -> Small
- B -> Small
- C -> Medium
- D -> Large