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/plugins/base/lang/en/lang.yaml

@@ -0,0 +1,3 @@
+greeting: Hello
+labels:
+  pluginName: Hello plugin

examples/hello/plugins/base/lang/pl/lang.yaml

@@ -0,0 +1,3 @@
+greeting: Cześć
+labels:
+  pluginName: Wtyczka Hello