refactor: migració a l'arquitectura SDL3 Callback API

Substitueix el bucle blocant main() → Director::run() → escena::run() per
SDL_AppInit/Iterate/Event/Quit. Cada escena implementa ara iterate() (un frame)
i handleEvent() (un event) sota una interfície base Scene.

- Director gestiona l'escena activa i les transicions via switchToActiveScene()
- Setup/cleanup que estava al voltant del while de run() mogut a ctor/dtor
  (música de Game/Ending/Ending2, volum de LoadingScreen)
- GlobalEvents ja no processa SDL_EVENT_QUIT (ho fa Director::handleEvent)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

source/main.cpp

@@ -5,14 +5,24 @@ Empezado en Castalla el 01/07/2022.
 
 */
 
-#include <memory>
+#define SDL_MAIN_USE_CALLBACKS 1
+#include <SDL3/SDL_main.h>
 
 #include "core/system/director.hpp"
 
-auto main() -> int {
-    // Crea el objeto Director
-    auto director = std::make_unique<Director>();
-
-    // Bucle principal
-    return Director::run();
+SDL_AppResult SDL_AppInit(void** appstate, int /*argc*/, char* /*argv*/[]) {
+    *appstate = new Director();
+    return SDL_APP_CONTINUE;
+}
+
+SDL_AppResult SDL_AppIterate(void* appstate) {
+    return static_cast<Director*>(appstate)->iterate();
+}
+
+SDL_AppResult SDL_AppEvent(void* appstate, SDL_Event* event) {
+    return static_cast<Director*>(appstate)->handleEvent(*event);
+}
+
+void SDL_AppQuit(void* appstate, SDL_AppResult /*result*/) {
+    delete static_cast<Director*>(appstate);
 }