feat: port forwards are a firewall reader too, and only the WAN's

get_port_forwards was declared on ResidentialGatewayDriver alone, as if a
port forward were a home-router feature. A firewall forwards ports just the
same (OPNsense calls it destination NAT), and netOrk asks both: is this host
reachable from the internet, which CVEs are exposed. The declaration moves to
NatVpnMixin, where the two roles already overlap, and PortForwardDict next to
NATTranslationDict.

The contract now says what counts. Destination NAT between internal networks
and rules that only exempt traffic are not port forwards: callers read every
entry as "reachable from outside". "ANY" forwards every protocol and an
external port of 0 every port -- a whole host forwarded is the most exposed
case and must not fall out for lack of a port number.

Declaration only, under TYPE_CHECKING: nothing changes at runtime.

README.md

@@ -227,6 +227,11 @@ class PfSenseDriver(FirewallDriver):
     def get_vpn_tunnels(self):
         # return Dict[str, VPNTunnelDict]
         ...
+
+    def get_port_forwards(self):
+        # return List[PortForwardDict] — forwards from the WAN only, never a
+        # redirect between internal networks (shared with home gateways)
+        ...
 ```
 
 ### Hypervisor