refactor: fase 5 — singletons a std::unique_ptr (elimina new/delete manual)

5 singletons afectats: Audio, Screen, Director, Resource::Cache, Resource::List.

- static T* instance → static std::unique_ptr<T> instance
- init(): new T() adoptat immediatament per unique_ptr (ownership RAII)
- destroy(): instance.reset() (sense delete manual)
- get(): retorna instance.get()
- Destructors moguts a public perquè std::default_delete hi pugui accedir
  (ctors privats + copy/move deleted → encapsulació efectiva mantinguda)

Ordre de destrucció preservat: SDL_AppQuit segueix cridant destroy() en
l'ordre invers a init() — la RAII automàtica no s'activa fins al final
del programa (LIFO de variables static).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

source/core/rendering/screen.cpp

@@ -55,19 +55,18 @@ namespace {
 }  // namespace
 #endif  // __EMSCRIPTEN__
 
-Screen* Screen::instance_ = nullptr;
+std::unique_ptr<Screen> Screen::instance_;
 
 void Screen::init() {
-    instance_ = new Screen();
+    instance_ = std::unique_ptr<Screen>(new Screen());
 }
 
 void Screen::destroy() {
-    delete instance_;
-    instance_ = nullptr;
+    instance_.reset();
 }
 
 auto Screen::get() -> Screen* {
-    return instance_;
+    return instance_.get();
 }
 
 Screen::Screen() {