test(nano): cover the nano delivery path and its loopback default

felis nano serves the same hasJoined handler as felis api, but behind
nanoStubRepo, which implements only the bar-list lookup and embeds a nil
Repo for everything else. Only the full-api path was tested, against a
complete fake store, so a second store call added to handleHasJoined
would pass CI and panic on every nano login. The loopback default of
-listen, the one thing keeping nano from being an open auth relay, was
not pinned either.

The default moves into a nanoDefaultListen constant, and two tests
cover the path. One serves a login through api.HasJoinedHandler with
nanoStubRepo and a fake identity source and expects the profile back.
The other requires the default to parse as a loopback IP. Taking the
bar-list method off the stub makes the first panic on the nil Repo;
defaulting to 0.0.0.0:8081 or :8081 fails the second.

cmd/felis/nano_test.go

@@ -14,6 +14,8 @@ import (
 	"testing"
 	"time"
 	"unicode/utf8"
+
+	"felis.lolicon.best/internal/api"
 )
 
 // [server] listen in a nano config reads like the bind address but is not one; nano must
@@ -85,6 +87,33 @@ func TestNanoDrainsInFlightLoginOnShutdown(t *testing.T) {
 	}
 }
 
+// The nano delivery path: the shared handler behind nano's stub store must admit a login its
+// source validated. nanoStubRepo implements only the bar-list lookup, so a new store call in
+// handleHasJoined would reach its nil embedded Repo and panic here, while the full-api tests,
+// which use a complete fake store, stay green.
+func TestNanoAdmitsAValidatedLogin(t *testing.T) {
+	const id = "069a79f444e94726a5befca90e38aaf5"
+	ygg := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"id":"`+id+`","name":"Notch"}`)
+	}))
+	defer ygg.Close()
+	h := api.HasJoinedHandler([]api.AuthSource{{Tag: "mojang", URL: ygg.URL, Identity: true}}, nanoStubRepo{})
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, httptest.NewRequest(http.MethodGet, "/session/minecraft/hasJoined?username=Notch&serverId=abc", nil))
+	if w.Code != http.StatusOK || !strings.Contains(w.Body.String(), id) {
+		t.Fatalf("code = %d body = %q, want the validated profile", w.Code, w.Body.String())
+	}
+}
+
+// An unauthenticated relay on a public address spends this host's Mojang rate limit for
+// anyone who finds it, so the default bind has to stay loopback.
+func TestNanoListensOnLoopbackByDefault(t *testing.T) {
+	host, _, err := net.SplitHostPort(nanoDefaultListen)
+	if ip := net.ParseIP(host); err != nil || ip == nil || !ip.IsLoopback() {
+		t.Fatalf("default -listen %q is not a loopback address", nanoDefaultListen)
+	}
+}
+
 // The request log prints text the caller chose. A bidi override must not reorder the line,
 // an invalid byte must not make journald store the entry as a blob, and a huge query must
 // not become a huge log line. serverId is left out so the handler answers without asking