```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 must be greater than zero.")
    
    slots = []
    for i in range(0, len(items), capacity):
        if i + capacity >= len(items):
            slots.append(items[i:])
        else:
            slots.append(items[i:i + capacity])
    return slots
```