```python
"""Vet a device's proposed firmware upgrade path and report the final tag."""


def vet_upgrade_path(installed: str, steps: list) -> str:
    carried = installed
    for step in steps:
        if not isinstance(step, dict):
            raise ValueError(f"step {step} is not a dictionary")
        if not isinstance(step["tag"], str):
            raise ValueError(f"step {step['tag']} is not a string")
        if not step["tag"].isdigit() or len(step["tag"].split(".")) != 2:
            raise ValueError(f"step {step['tag']} is not a valid tag")
        tag_parts = step["tag"].split(".")
        if any(part[0] == "0" for part in tag_parts):
            raise ValueError(f"step {step['tag']} contains leading zeros")
        if len(tag_parts) != 2:
            raise ValueError(f"step {step['tag']} is not a two-part tag")
        line, point = map(int, tag_parts)
        if carried >= step["requires"]:
            raise ValueError(f"step {step['tag']} is not an upgrade from {carried}")
        carried = step["tag"]
    return carried
```