```python
def trace_relay(links: dict, start: str) -> list:
    if not isinstance(links, dict):
        raise ValueError("links must be a mapping")
    if start not in links:
        raise ValueError(f"start post '{start}' is not named in links")
    
    visited = set()
    current_post = start
    path = []
    
    while current_post != "":
        if current_post in visited:
            raise ValueError(f"handoff loop detected: {current_post} already visited")
        visited.add(current_post)
        path.append(current_post)
        current_post = links[current_post]
    
    return path
```