```python
"""Walks a courier relay directory station by station."""


def trace_relay(links: dict, start: str) -> list:
    if not isinstance(start, str):
        raise ValueError("start must be a string")
    if start == "":
        raise ValueError("start cannot be an empty string")
    if start not in links:
        raise ValueError("unknown starting station")

    path = []
    visited = set()
    current = start
    while True:
        if current in visited:
            raise ValueError("circular relay detected")
        path.append(current)
        visited.add(current)
        target = links[current]
        if target == "":
            break
        if not isinstance(target, str):
            raise ValueError("link value must be a string")
        if target not in links:
            raise ValueError(f"unknown station {target} in link")
        current = target

    return path
```