feat: add ghost mode grayscale screen effect

- FXAA path: repurpose _pad field as 'desaturate' push constant; when
  ghostMode_ is true, convert final pixel to grayscale with slight cool
  blue tint using luma(0.299,0.587,0.114) mix
- Non-FXAA path: apply a high-opacity gray overlay (rgba 0.5,0.5,0.55,0.82)
  over the scene for a washed-out look
- Both parallel (SEC_POST) and single-threaded render paths covered
- ghostMode_ flag set each frame from gameHandler->isPlayerGhost()

include/rendering/renderer.hpp

@@ -638,6 +638,8 @@ private:
     bool terrainEnabled = true;
     bool terrainLoaded = false;
 
+    bool ghostMode_ = false;  // set each frame from gameHandler->isPlayerGhost()
+
     // CPU timing stats (last frame/update).
     double lastUpdateMs = 0.0;
     double lastRenderMs = 0.0;