feat: integrate lightning system for storm weather and heavy rain

The lightning system (lightning.hpp/cpp) was fully implemented but never
wired into the renderer. Connect it now:
- Enable lightning during server storm weather (wType==3, intensity>0.1)
  and heavy rain (wType==1, intensity>0.7) as a bonus visual
- Scale lightning intensity proportionally to weather intensity
- Render in both parallel (SEC_POST) and fallback rendering paths
- Update and shutdown alongside the weather system
- Show active lightning info in the performance HUD weather section

src/rendering/performance_hud.cpp

@@ -10,6 +10,7 @@
 #include "rendering/clouds.hpp"
 #include "rendering/lens_flare.hpp"
 #include "rendering/weather.hpp"
+#include "rendering/lightning.hpp"
 #include "rendering/character_renderer.hpp"
 #include "rendering/wmo_renderer.hpp"
 #include "rendering/m2_renderer.hpp"
@@ -369,6 +370,11 @@ void PerformanceHUD::render(const Renderer* renderer, const Camera* camera) {
                 ImGui::Text("Intensity: %.0f%%", weather->getIntensity() * 100.0f);
             }
 
+            auto* lightning = renderer->getLightning();
+            if (lightning && lightning->isEnabled()) {
+                ImGui::Text("Lightning: active (%.0f%%)", lightning->getIntensity() * 100.0f);
+            }
+
             ImGui::Spacing();
         }
     }