fix: retain notifications while the scene background is absent

Buffer reward messages until the shell returns without allocating detached UI nodes. Register mod tests that verify notification ordering and prevent duplicates.

manifest.json

@@ -9,6 +9,9 @@
   "sources": [
     "src/**/*.cs"
   ],
+  "test_sources": [
+    "tests/*.cs"
+  ],
   "ui": [
     {
       "id": "game_menu",