feat(stream): add NAT-PMP port mapping for remote downloads

Replace anacrolix/upnp with huin/goupnp + custom NAT-PMP (RFC 6886)
implementation. NAT-PMP is tried first (faster, more compatible with
TP-Link routers), with UPnP-IGD SOAP as fallback. Gateway detection
reads /proc/net/route for accuracy. Includes unit tests with mock
NAT-PMP server and permanent e2e tests (build tag manual).

internal/engine/watch_reporter_test.go

@@ -104,6 +104,7 @@ func TestStreamServerRangeTracking(t *testing.T) {
 	}
 
 	srv := NewStreamServerFromDisk(tmpFile, 0)
+	srv.disableUPnP = true
 	ctx := context.Background()
 	url, err := srv.Start(ctx)
 	if err != nil {