test(net): e2e-test the UPnP path + add a portmap diagnostic

Address the gap that the port mapper's UPnP path and gateway discovery
had no automated coverage (only NAT-PMP did):

- UPnP e2e: split map_via_gateway out of discovery and drive it through
  igd-next against an in-process mock IGD (a std HTTP server returning a
  canned SOAP envelope). Verifies the real add_port SOAP round-trip and
  that we request the exact external/internal port + LAN client.
- default_gateway parsing is now pure, testable functions with unit
  tests for the macOS and Linux formats (including the 0.0.0.0 skip).
- New 'portmap' CLI subcommand (needs no login): synchronously probes
  UPnP/NAT-PMP against the real router and reports whether the port was
  opened + the external address, then removes the test mapping — so a
  user can verify reachability on their own network.

9 port_mapping tests now cover the NAT-PMP protocol, the UPnP SOAP
round-trip, gateway parsing, and the map/renew/cleanup lifecycle.

soulseek-rs/src/cli.rs

@@ -120,6 +120,9 @@ pub enum Commands {
         #[arg(short, long, default_value = "30")]
         listen_secs: u64,
     },
+
+    /// Test whether your router lets us auto-open the listen port (UPnP/NAT-PMP)
+    Portmap,
 }
 
 pub fn parse_server_address(server: &str) -> color_eyre::Result<(String, u16)> {