feat(04-02): load namespaced translations from embedded plugins

- Walk HasLang FS for lang/<locale>/<group>.yaml and flatten nested keys
- Publish one phrasebook.Translator before Boot with en/en defaults
- Hello plugin resolves en/pl vendor.plugin::group.key after Activate

Co-authored-by: Cursor <cursoragent@cursor.com>

examples/hello/hello_test.go

@@ -15,6 +15,7 @@ import (
 	"git.golem15.com/golem15/summercms/compass"
 	"git.golem15.com/golem15/summercms/pact"
 	"git.golem15.com/golem15/summercms/party"
+	"git.golem15.com/golem15/summercms/phrasebook"
 	"git.golem15.com/golem15/summercms/surf"
 )
 
@@ -107,6 +108,30 @@ func TestBuiltBinaryGreeterHello(t *testing.T) {
 	assertGreeting(t, got, want)
 }
 
+func TestHelloPluginResolvesLang(t *testing.T) {
+	cfg, err := compass.Load("config")
+	if err != nil {
+		t.Fatal(err)
+	}
+	app := backpack.New(cfg)
+	if _, err := party.Activate(app, []string{"golem15.hello"}); err != nil {
+		t.Fatal(err)
+	}
+	tr, ok := app.Lookup[*phrasebook.Translator]()
+	if !ok {
+		t.Fatal("translator not published")
+	}
+	if got := tr.GetIn("en", "golem15.hello::lang.labels.pluginName", nil); got != "Hello plugin" {
+		t.Fatalf("en = %q", got)
+	}
+	if got := tr.GetIn("pl", "golem15.hello::lang.labels.pluginName", nil); got != "Wtyczka Hello" {
+		t.Fatalf("pl = %q", got)
+	}
+	if got := tr.GetIn("en", "golem15.hello::lang.greeting", nil); got != "Hello" {
+		t.Fatalf("greeting = %q", got)
+	}
+}
+
 func TestTypedItemRoute(t *testing.T) {
 	cfg, err := compass.Load("config")
 	if err != nil {