fix: replace HSV df_fade_color with exact stacked blue compositing

src/world/tiles/tilemap_chunk.rs

@@ -86,8 +86,18 @@ fn df_fade_color(z_diff: i32) -> Color {
     if z_diff <= 0 {
         return Color::WHITE;
     }
-    let saturation = (z_diff as f32 / 8.0).clamp(0.0, 1.0);
-    Color::hsv(194.7, saturation, 1.0 - saturation / 2.0)
+    let n = (z_diff as f32).min(8_f32);
+    let overlay_r = 133_f32 / 255_f32;
+    let overlay_g = 167_f32 / 255_f32;
+    let overlay_b = 178_f32 / 255_f32;
+    let layer_alpha = 41_f32 / 255_f32;
+    let acc_alpha = 1_f32 - (1_f32 - layer_alpha).powf(n);
+    let inv = 1_f32 - acc_alpha;
+    Color::srgb(
+        overlay_r * acc_alpha + inv,
+        overlay_g * acc_alpha + inv,
+        overlay_b * acc_alpha + inv,
+    )
 }
 
 fn is_tile_visible_at_z(tile_data: &FloorTileData, z_index: usize) -> bool {