```python
from typing import List, Union

def fork_paths(pattern: str) -> List[str]:
    if not pattern or pattern.isspace():
        raise ValueError("Input cannot be empty or contain only whitespace.")
    
    def is_fork_segment(segment):
        return segment.startswith('{') and segment.endswith('}')
    
    def split_fork_segment(segment):
        return segment[1:-1].split(',')
    
    paths = []
    current_path = []
    
    for char in pattern:
        if char == '/':
            if current_path:
                paths.append(''.join(current_path))
                current_path = []
        elif char == '{':
            current_path.append('')
        elif char == ',':
            if current_path and is_fork_segment(current_path[-1]):
                current_path[-1] += char
            else:
                current_path.append('')
        elif char == '}':
            if current_path and is_fork_segment(current_path[-1]):
                current_path[-1] += char
                fork_options = split_fork_segment(current_path.pop())
                for option in fork_options:
                    current_path.append(option)
            else:
                current_path.append('')
        else:
            if is_fork_segment(current_path[-1]):
                current_path[-1] += char
            else:
                current_path.append(char)
    
    if current_path:
        paths.append(''.join(current_path))
    
    return paths
```