Fix: F5 (toggle scaling) respeta resolución personalizada

Problema:
- Con -w 960 -h 800, al pulsar F5 en fullscreen (F3)
- La resolución cambiaba a 320x240 hardcoded
- Pantalla quedaba rota con viewport diminuto

Causa:
toggleIntegerScaling() usaba SCREEN_WIDTH/HEIGHT constantes
en SDL_SetRenderLogicalPresentation()

Solución:
Usar current_screen_width_/height_ dinámicos

Ahora funciona:
  ./vibe3_physics -w 960 -h 800
  F3 (fullscreen) → OK ✅
  F5 (cambiar scaling) → OK ✅
  ESC (salir fullscreen) → OK ✅

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

source/engine.cpp

@@ -954,7 +954,7 @@ void Engine::toggleIntegerScaling() {
             break;
     }
 
-    SDL_SetRenderLogicalPresentation(renderer_, SCREEN_WIDTH, SCREEN_HEIGHT, presentation);
+    SDL_SetRenderLogicalPresentation(renderer_, current_screen_width_, current_screen_height_, presentation);
 
     // Mostrar texto informativo
     text_ = "SCALING: ";