fix: handle non-canonical CIDR notation where host bits are set

[Nicceboy]

Problem

RustScan fails silently when a CIDR is given where the input IP has host bits set (i.e. the IP is not the network address for that prefix).

rustscan -a 192.168.1.13/29
[!] Host "192.168.1.13/29" could not be resolved.
[!] No IPs could be resolved, aborting scan.

This is valid and common input. Users naturally provide their known host IP with a subnet mask to mean "scan this subnet". Tools like nmap accept it without complaint.

What currently happens:

1. IpInet::from_str("192.168.1.13/29") rejects it — host bits 101 in .13 are non-zero
2. Falls through to DNS resolution, which also fails
3. Address is treated as a file path, which also fails
4. A misleading "could not be resolved" warning is shown

Fix

Adds a normalize_cidr helper that masks off the host bits to derive the true network address before parsing. 192.168.1.13/29 becomes 192.168.1.8/29 (.13 = 0000 1101 → mask last 3 bits → .8 = 0000 1000), which is then expanded to the expected 8 addresses.

The change is one helper function and one additional else if branch in parse_address.