feat(cabana): settings entries that link to an admin controller

pact.SettingsItem gains an additive Controller field, the equivalent of a
WinterCMS registerSettings 'url' => Backend::url(...) entry. A link entry
declares no Model, Form or NewModel and needs no AdminFS; start-up fails
when it combines Controller with a singleton form or a model, or names an
unregistered controller. Settings codes stay one namespace.

GET /settings lists a link entry with its controller only when the
principal passes the item's permissions and may open the controller.
Registry.Setting never returns a link entry, so the singleton settings
endpoints answer 404 for its code. SettingsEntry carries controller,
empty for singletons; the OpenAPI document, generated SPA types and
settings fixture follow, and the pact and cabana READMEs and the
settings docs describe the link.

modules/cabana/example_test.go

@@ -101,3 +101,20 @@ func TestDocsDeclarations(t *testing.T) {
 		t.Fatal("BlogSettings has no posts_per_page rule")
 	}
 }
+
+// TestDocsSettingsLink activates the plugin with a settings entry that links
+// to its posts controller, as the Settings page shows.
+func TestDocsSettingsLink(t *testing.T) {
+	cfg, err := compass.Open(compass.Options{Dir: "config", Env: "development", Environ: []string{}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	_ = cfg.Set("admin.jwt.secret", "test-only-secret-with-at-least-32-bytes")
+	if _, err := cabana.Activate(backpack.New(cfg), []party.Plugin{&LinkedBlogPlugin{}}); err != nil {
+		t.Fatal(err)
+	}
+	settings := (&LinkedBlogPlugin{}).Settings()
+	if len(settings) != 2 || settings[1].Controller != "acme.blog.posts" || settings[1].Model != "" {
+		t.Fatalf("Settings() = %+v", settings)
+	}
+}