fix(engine): implementar viewport/scissor F6 i eliminar early return toggleIntegerScaling

- Eliminar guarda !fullscreen_enabled_ de toggleIntegerScaling(): F6 ara
  cicla i notifica en mode ventana i fullscreen per igual
- Pass 2: afegir viewport+scissor SDL_GPU condicionat a fullscreen_enabled_
  per als tres modes (INTEGER pixel-perfect centrat, LETTERBOX aspect-ratio,
  STRETCH pantalla completa)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

source/engine.cpp

@@ -857,6 +857,36 @@ void Engine::render() {
 
         SDL_GPURenderPass* pass2 = SDL_BeginGPURenderPass(cmd, &ct, 1, nullptr);
 
+        // Viewport/scissor per integer scaling (only F3 fullscreen)
+        if (fullscreen_enabled_) {
+            float vp_x, vp_y, vp_w, vp_h;
+            if (current_scaling_mode_ == ScalingMode::STRETCH) {
+                vp_x = 0.0f; vp_y = 0.0f;
+                vp_w = static_cast<float>(sw_w);
+                vp_h = static_cast<float>(sw_h);
+            } else if (current_scaling_mode_ == ScalingMode::INTEGER) {
+                int scale = static_cast<int>(std::min(sw_w / static_cast<Uint32>(base_screen_width_),
+                                                      sw_h / static_cast<Uint32>(base_screen_height_)));
+                if (scale < 1) scale = 1;
+                vp_w = static_cast<float>(base_screen_width_  * scale);
+                vp_h = static_cast<float>(base_screen_height_ * scale);
+                vp_x = (static_cast<float>(sw_w) - vp_w) * 0.5f;
+                vp_y = (static_cast<float>(sw_h) - vp_h) * 0.5f;
+            } else { // LETTERBOX
+                float scale = std::min(static_cast<float>(sw_w) / base_screen_width_,
+                                       static_cast<float>(sw_h) / base_screen_height_);
+                vp_w = base_screen_width_  * scale;
+                vp_h = base_screen_height_ * scale;
+                vp_x = (static_cast<float>(sw_w) - vp_w) * 0.5f;
+                vp_y = (static_cast<float>(sw_h) - vp_h) * 0.5f;
+            }
+            SDL_GPUViewport vp = {vp_x, vp_y, vp_w, vp_h, 0.0f, 1.0f};
+            SDL_SetGPUViewport(pass2, &vp);
+            SDL_Rect scissor = {static_cast<int>(vp_x), static_cast<int>(vp_y),
+                                static_cast<int>(vp_w),  static_cast<int>(vp_h)};
+            SDL_SetGPUScissor(pass2, &scissor);
+        }
+
         // PostFX: full-screen triangle via vertex_id (no vertex buffer needed)
         SDL_BindGPUGraphicsPipeline(pass2, gpu_pipeline_->postfxPipeline());
         SDL_GPUTextureSamplerBinding scene_tsb = {offscreen_tex_->texture(), offscreen_tex_->sampler()};
@@ -1060,11 +1090,6 @@ void Engine::setInitialPostFX(int mode) {
 }
 
 void Engine::toggleIntegerScaling() {
-    // Solo permitir cambio si estamos en modo fullscreen normal (F3)
-    if (!fullscreen_enabled_) {
-        return;  // No hacer nada si no estamos en fullscreen
-    }
-
     // Ciclar entre los 3 modos: INTEGER → LETTERBOX → STRETCH → INTEGER
     switch (current_scaling_mode_) {
         case ScalingMode::INTEGER:
@@ -1078,16 +1103,13 @@ void Engine::toggleIntegerScaling() {
             break;
     }
 
-    // SDL_GPU stretches to fill swapchain by default; just show notification
     const char* mode_name = "INTEGER";
     switch (current_scaling_mode_) {
         case ScalingMode::INTEGER:   mode_name = "INTEGER";   break;
         case ScalingMode::LETTERBOX: mode_name = "LETTERBOX"; break;
         case ScalingMode::STRETCH:   mode_name = "STRETCH";   break;
     }
-
+    showNotificationForAction(std::string("Escalado: ") + mode_name);
-    std::string notification = std::string("Escalado: ") + mode_name;
-    ui_manager_->showNotification(notification);
 }
 
 void Engine::addSpriteToBatch(float x, float y, float w, float h, int r, int g, int b, float scale) {