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);
 }

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;

src/rendering/renderer.cpp

@@ -5044,7 +5044,7 @@ void Renderer::renderFXAAPass() {
     vkCmdBindDescriptorSets(currentCmd, VK_PIPELINE_BIND_POINT_GRAPHICS,
                             fxaa_.pipelineLayout, 0, 1, &fxaa_.descSet, 0, nullptr);
 
-    // Pass rcpFrame + sharpness (vec4, 16 bytes).
+    // Pass rcpFrame + sharpness + desaturate (vec4, 16 bytes).
     // When FSR2/FSR3 is active alongside FXAA, forward FSR2's sharpness so the
     // post-FXAA unsharp-mask step restores the crispness that FXAA's blur removes.
     float sharpness = fsr2_.enabled ? fsr2_.sharpness : 0.0f;
@@ -5052,7 +5052,7 @@ void Renderer::renderFXAAPass() {
         1.0f / static_cast<float>(ext.width),
         1.0f / static_cast<float>(ext.height),
         sharpness,
-        0.0f
+        ghostMode_ ? 1.0f : 0.0f  // desaturate: 1=ghost grayscale, 0=normal
     };
     vkCmdPushConstants(currentCmd, fxaa_.pipelineLayout,
                        VK_SHADER_STAGE_FRAGMENT_BIT, 0, 16, pc);
@@ -5092,6 +5092,9 @@ void Renderer::renderWorld(game::World* world, game::GameHandler* gameHandler) {
     lastWMORenderMs = 0.0;
     lastM2RenderMs = 0.0;
 
+    // Cache ghost state for use in overlay and FXAA passes this frame.
+    ghostMode_ = (gameHandler && gameHandler->isPlayerGhost());
+
     uint32_t frameIdx = vkCtx->getCurrentFrame();
     VkDescriptorSet perFrameSet = perFrameDescSets[frameIdx];
     const glm::mat4& view = camera ? camera->getViewMatrix() : glm::mat4(1.0f);
@@ -5237,6 +5240,12 @@ void Renderer::renderWorld(game::World* world, game::GameHandler* gameHandler) {
                     renderOverlay(tint, cmd);
                 }
             }
+            // Ghost mode desaturation overlay (non-FXAA path approximation).
+            // When FXAA is active the FXAA shader applies true per-pixel desaturation;
+            // otherwise a high-opacity gray overlay gives a similar washed-out effect.
+            if (ghostMode_ && overlayPipeline && !fxaa_.enabled) {
+                renderOverlay(glm::vec4(0.5f, 0.5f, 0.55f, 0.82f), cmd);
+            }
             if (minimap && minimap->isEnabled() && camera && window) {
                 glm::vec3 minimapCenter = camera->getPosition();
                 if (cameraController && cameraController->isThirdPerson())
@@ -5369,6 +5378,10 @@ void Renderer::renderWorld(game::World* world, game::GameHandler* gameHandler) {
                 renderOverlay(tint);
             }
         }
+        // Ghost mode desaturation overlay (non-FXAA path approximation).
+        if (ghostMode_ && overlayPipeline && !fxaa_.enabled) {
+            renderOverlay(glm::vec4(0.5f, 0.5f, 0.55f, 0.82f));
+        }
         if (minimap && minimap->isEnabled() && camera && window) {
             glm::vec3 minimapCenter = camera->getPosition();
             if (cameraController && cameraController->isThirdPerson())