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


def vet_upgrade_path(installed: str, steps: list) -> str:
    def parse_tag(tag: str) -> tuple[int, int]:
        parts = tag.split('.')
        if len(parts) != 2 or not parts[0].isdigit() or not parts[1].isdigit():
            raise ValueError(f"Invalid tag format: {tag}")
        line, point = map(int, parts)
        if line < 0 or point < 0:
            raise ValueError(f"Negative version number: {tag}")
        return line, point

    carried = parse_tag(installed)
    for step in steps:
        if not isinstance(step, dict) or 'tag' not in step or 'requires' not in step:
            raise ValueError("Invalid step format")
        requires_tag = parse_tag(step['requires'])
        step_tag = parse_tag(step['tag'])
        if requires_tag >= step_tag:
            raise ValueError(f"step {step['tag']} requires at least {step['requires']}")
        if step_tag <= carried:
            raise ValueError(f"step {step['tag']} is not an upgrade from {carried}")
        carried = step_tag
    return '.'.join(map(str, carried))
```