Loading screen between world setup and the game scene

World generation (WorldScene.OnLoad: terrain, atlases, spawn) runs synchronously
and blocks the frame; previously only a blank fade covered it, with no feedback.
New WorldLoadingScene shows a "Generating world" label, lets it draw, then switches
to WorldScene WITHOUT a transition so its last drawn frame (the label) stays on
screen during the blocking generation. Both New World and Load Game route through
it. Localized loading.world ru/en. Build + --check-content clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Mods/Core/Languages/en/ui.json

@@ -92,6 +92,7 @@
   "menu.settings": "Settings",
   "menu.credits": "Credits",
   "menu.quit": "Quit",
+  "loading.world": "Generating world",
   "newworld.title": "New World",
   "newworld.name": "Name",
   "newworld.defaultname": "New World",

Mods/Core/Languages/ru/ui.json

@@ -92,6 +92,7 @@
   "menu.settings": "Настройки",
   "menu.credits": "Авторы",
   "menu.quit": "Выход",
+  "loading.world": "Создание мира",
   "newworld.title": "Новый мир",
   "newworld.name": "Название",
   "newworld.defaultname": "Новый мир",