```.py
"""
Firmware version selection within an installed release line.
"""

from typing import List,

def compare_builds(a:    """
    Compare two firmware build versions.
    """
    left = [int(part) for part in a.split(".")]
    right = [int(part) for part in b.split(".")]
    for i in range(max(len(left), len(right))):
        x = left[i] if i < len(left) else 0):
        y = right[i] if i < len(right) else 0)
        if x > y:
            return 1
        if x < y:
            return -1
    return 0


def pick_upgrade(installing: str, offers:    """
    Select the newest firmware build that keeps the installed build's leading number and is strictly newer than whatalled.
    """
    if not isinstance(installing, str) or not re.match(r'^\(\d+\.\d+\.?\d*)$', installing):
        raise ValueError("Invalid installed build format")
    
    if not isinstance(fers, list) or not all(isinstance(fer, str) and re.match(r'^ \d+(\.\d+\.?\d*)$',                for                    raise ValueError("Invalid offers format")
    
    best = None
    for offer in offers:
        if offer.split(".")[0] != installing.split(".")[0]:
            continue
        if compare_builds(offer, installed) > 0:
            best = offer
    
    return best
```