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()

assets/shaders/fxaa.frag.glsl

@@ -2,7 +2,7 @@
 
 // FXAA 3.11 — Fast Approximate Anti-Aliasing post-process pass.
 // Reads the resolved scene color and outputs a smoothed result.
-// Push constant: rcpFrame = vec2(1/width, 1/height), sharpness (0=off, 2=max), unused.
+// Push constant: rcpFrame = vec2(1/width, 1/height), sharpness (0=off, 2=max), desaturate (1=ghost grayscale).
 
 layout(set = 0, binding = 0) uniform sampler2D uScene;
 
@@ -11,8 +11,8 @@ layout(location = 0) out vec4 outColor;
 
 layout(push_constant) uniform PC {
     vec2  rcpFrame;
-    float sharpness;  // 0 = no sharpen, 2 = max (matches FSR2 RCAS range)
-    float _pad;
+    float sharpness;    // 0 = no sharpen, 2 = max (matches FSR2 RCAS range)
+    float desaturate;   // 1 = full grayscale (ghost mode), 0 = normal color
 } pc;
 
 // Quality tuning
@@ -145,5 +145,11 @@ void main() {
         fxaaResult = clamp(fxaaResult + s * (fxaaResult - blur), 0.0, 1.0);
     }
 
+    // Ghost mode: desaturate to grayscale (with a slight cool blue tint).
+    if (pc.desaturate > 0.5) {
+        float gray = dot(fxaaResult, vec3(0.299, 0.587, 0.114));
+        fxaaResult = mix(fxaaResult, vec3(gray, gray, gray * 1.05), pc.desaturate);
+    }
+
     outColor = vec4(fxaaResult, 1.0);
 }