Function classify_interface 

pub fn classify_interface(
    addr: &IpAddr,
    config: &InterfaceConfig,
) -> NetInterface

Classifies an IP address into the NetInterface taxonomy.

Order:

1. Loopback (127/8, ::1).
2. Named matches from config.named (first match wins).
3. Local-subnet matches from config.local_subnets.
4. Fallback Wan.