IPv4 CIDR Utilities

Implement IPv4 network helpers without the ipaddress module:

• ip_to_int(ip) and int_to_ip(n): convert between dotted quads and 32-bit integers, raising ValueError for malformed addresses (wrong part count, non-numeric parts, values over 255)
• parse_cidr(cidr): "10.1.0.0/16" → (network_int, prefix). Raise ValueError if host bits are set (like "10.1.2.3/16") or the prefix is outside 0–32.
• contains(cidr, ip)
• subnets(cidr, new_prefix): split into the list of smaller CIDR strings
• collapse(cidrs): the minimal sorted list of CIDRs covering exactly the union, by removing covered networks and repeatedly merging adjacent sibling pairs (two /25s that make up a /24)