```python
def slot_pack(items: list, capacity: int) -> list:
    """Group items into slots of a fixed capacity, in order."""
    if capacity <= 0:
        raise ValueError("Capacity must be greater than zero.")
    
    slots = []
    i = 0
    while i < len(items):
        slots.append(items[i : i + capacity])
        i += capacity
    
    return slots
```