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


def trace_relay(links: dict, start: str) -> list:
    """Walks a courier relay directory station by station."""
    if not isinstance(start, str):
        raise ValueError("start must be a string")
    if start not in links:
        raise ValueError("unknown starting station")
    if "" in links:
        raise ValueError("empty station name in links")

    path = []
    visited = set()
    current = start

    while True:
        if current in visited:
            raise ValueError("circular relay detected")
        visited.add(current)
        path.append(current)

        target = links[current]
        if target == "":
            break

        if not isinstance(target, str):
            raise ValueError("link target must be a string")
        if target not in links:
            raise ValueError("unknown next station")

        current = target

    return path
```