feat(rendering): add fog-distance darkness to chunk shaders

Assets/shaders/chunk.frag

@@ -1,13 +1,20 @@
 #version 330 core
 in vec2 vUv;
 in float vShade;
+in float vWorldY;
 
 out vec4 FragColor;
 
 uniform sampler2D atlas;
+uniform float u_caveDarkStartY;
+uniform float u_caveDarkEndY;
+uniform float u_caveMinBrightness;
 
 void main()
 {
     vec4 texel = texture(atlas, vUv);
-    FragColor = vec4(texel.rgb * vShade, 1.0);
+    float t = clamp((u_caveDarkStartY - vWorldY) /
+                    (u_caveDarkStartY - u_caveDarkEndY), 0.0, 1.0);
+    float brightness = mix(1.0, u_caveMinBrightness, t);
+    FragColor = vec4(texel.rgb * vShade * brightness, 1.0);
 }

Assets/shaders/chunk.vert

@@ -5,6 +5,7 @@ layout(location = 2) in float aShade;
 
 out vec2 vUv;
 out float vShade;
+out float vWorldY;
 
 uniform mat4 model;
 uniform mat4 view;
@@ -14,6 +15,8 @@ void main()
 {
     vUv = aUv;
     vShade = aShade;
-    // Vector-on-left convention; matrices are uploaded with transpose=true.
-    gl_Position = vec4(aPos, 1.0) * model * view * projection;
+    // World-space position uses row-vector convention (matches gl_Position below).
+    vec4 worldPos = vec4(aPos, 1.0) * model;
+    vWorldY = worldPos.y;
+    gl_Position = worldPos * view * projection;
 }