```
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("start is not a post")

    route = [start]
    current_post = start

    while True:
        if current_post == "":
            break
        next_post = links[current_post]
        
        if next_post not in links:
            raise ValueError(f"Unknown handoff from {current_post} to {next_post}")
        
        if next_post in route:
            raise ValueError("Watch comes back to a post already visited")

        route.append(next_post)
        current_post = next_post

    return route
```