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


def vet_upgrade_path(installed: str, steps: list) -> str:
    def is_valid_tag(tag):
        parts = tag.split('.')
        if len(parts) != 2 or not all(part.isdigit() for part in parts):
            return False
        line, point = map(int, parts)
        return line >= 0 and point >= 0

    carried = installed
    for step in steps:
        if not isinstance(step, dict) or 'tag' not in step or 'requires' not in step:
            raise ValueError("Each step must be a {tag, requires} record")
        if not is_valid_tag(step['tag']) or not is_valid_tag(step['requires']):
            raise ValueError("Invalid tag format")
        line_step, point_step = map(int, step['tag'].split('.'))
        line_requires, point_requires = map(int, step['requires'].split('.'))
        if line_step < line_requires or (line_step == line_requires and point_step <= point_requires):
            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 carried
```