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/resources/resource_cache.cpp

@@ -20,14 +20,11 @@ extern unsigned char* LoadPalette(unsigned char* data);
 
 namespace Resource {
 
-    Cache* Cache::instance = nullptr;
+    std::unique_ptr<Cache> Cache::instance;
 
-    void Cache::init() { instance = new Cache(); }
-    void Cache::destroy() {
-        delete instance;
-        instance = nullptr;
-    }
-    auto Cache::get() -> Cache* { return instance; }
+    void Cache::init() { instance = std::unique_ptr<Cache>(new Cache()); }
+    void Cache::destroy() { instance.reset(); }
+    auto Cache::get() -> Cache* { return instance.get(); }
 
     namespace {
         auto basename(const std::string& path) -> std::string {