Returns true for a canonical CIDR range such as 10.0.0.0/8, with no host bits set.
Ambiguous forms (leading-zero or hex octets, IPv4-mapped IPv6) are rejected so client and
server agree on which network a rule matches.
Returns true when the low width - prefix host bits of addr are all zero. Requires
prefix <= width; the host_bits == 0 check also avoids u128::MAX >> 128, which panics.