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/system/director.cpp

@@ -35,7 +35,7 @@
 // Cheats del joc original — declarats a jinput.cpp
 extern void JI_moveCheats(Uint8 new_key);
 
-Director* Director::instance_ = nullptr;
+std::unique_ptr<Director> Director::instance_;
 
 Director::~Director() = default;
 
@@ -79,7 +79,7 @@ std::unique_ptr<scenes::Scene> Director::createNextScene() {
 }
 
 void Director::init() {
-    instance_ = new Director();
+    instance_ = std::unique_ptr<Director>(new Director());
     Gamepad::init();
 
     // Registre d'escenes. Cada entrada = un state_key (`num_piramide`)
@@ -116,12 +116,11 @@ void Director::init() {
 
 void Director::destroy() {
     Gamepad::destroy();
-    delete instance_;
-    instance_ = nullptr;
+    instance_.reset();
 }
 
 auto Director::get() -> Director* {
-    return instance_;
+    return instance_.get();
 }
 
 void Director::togglePause() {