```python
from typing import List

def slot_pack(items: List[int], capacity: int) -> List[List[int]]:
    """Group items into slots of a fixed capacity, in order."""
    if capacity == 0:
        raise ValueError("Capacity cannot be zero.")
    
    slots = []
    for i in range(0, len(items), capacity):
        slots.append(items[i : i + capacity])
    return slots
```