Activate WMO/char/M2 render loop, purge dead GL block, add underwater overlay

- renderWorld() now calls wmoRenderer, characterRenderer, m2Renderer (+smoke,
  particles) in the correct opaque→transparent order; water moved after all
  opaques; per-subsystem timing active in live path
- Deleted the 310-line #if 0 GL stub block and removed #include <GL/glew.h>
  (last GL reference in renderer.cpp)
- Full-screen overlay pipeline (postprocess.vert + overlay.frag, alpha blend,
  no depth test/write) for underwater tint; lazily initialized, renders a blue
  tint when camera is meaningfully below the water surface; canal vs open-water
  tint colours preserved from original design
- overlay.frag.glsl / overlay.frag.spv added

include/rendering/renderer.hpp

@@ -295,6 +295,12 @@ private:
     float selCircleRadius = 1.5f;
     bool selCircleVisible = false;
 
+    // Fullscreen color overlay (underwater tint)
+    VkPipeline overlayPipeline = VK_NULL_HANDLE;
+    VkPipelineLayout overlayPipelineLayout = VK_NULL_HANDLE;
+    void initOverlayPipeline();
+    void renderOverlay(const glm::vec4& color);
+
     // Footstep event tracking (animation-driven)
     uint32_t footstepLastAnimationId = 0;
     float footstepLastNormTime = 0.0f;