examples: apply real texture assets to multi-mesh examples

- meshes/cube: procedural checkerboard -> uv_texture.jpg (8x8 UV grid)
- meshes/pbr: floor -> ground.jpeg, bump cube -> cave.jpg + caveNormal.jpg
  (normal map pre-encoded via sRGB OETF to cancel the GPU sRGB decode)
- lights/shadow: ground -> ground.jpeg, cube -> uv_texture.jpg
- effects/demo: ground -> ground.jpeg, cube -> uv_texture.jpg
- effects/fog: ground -> ground.jpeg (tiled 80x80), cubes -> stonewall.jpg
- effects/dof: ground -> ground.jpeg, cubes -> uv_texture.jpg
- cameras/culling: shared cube mesh -> uv_texture.jpg
- add lib/examples/assets/textures/ (19 assets, 6.5 MB)
- document assets + usage in examples READMEs, docs/user/examples.md,
  docs/user/meshes/materials.md (CARGO_MANIFEST_DIR pattern, sRGB caveat)

lib/examples/effects/fog.rs

@@ -35,9 +35,13 @@ use wsg_lib::app::AppBuilder;
 use wsg_lib::camera::CameraController;
 use wsg_lib::core::{FogConfig, ToneMapper, Transform};
 use wsg_lib::mesh::{cube, icosphere, plane};
+use wsg_lib::resources::Texture;
 use wsg_lib::AppHandler;
 use wsg_lib::utils::WsgError;
 
+/// Texture asset directory, resolved against the crate root so the example works from any CWD.
+const TEXTURES: &str = concat!(env!("CARGO_MANIFEST_DIR"), "/examples/assets/textures");
+
 struct FogDemo {
     camera: CameraController,
 }
@@ -48,15 +52,34 @@ impl AppHandler for FogDemo {
             .register_shader("standard", wsg_lib::utils::STANDARD_SHADER_PATH)
             .unwrap();
 
+        // Textured ground + cubes: the fog falloff reads much better on textured surfaces.
+        // ground.jpeg tiles across the 80×80 floor (Repeat sampler); stonewall.jpg on the cubes.
+        let (device, queue) = {
+            let ctx = app.context();
+            (ctx.device.clone(), ctx.queue.clone())
+        };
+        let ground_tex =
+            Texture::from_file(&device, &queue, "ground", &format!("{TEXTURES}/ground.jpeg")).unwrap();
+        app.scene.add_texture("ground_texture", ground_tex).unwrap();
+        app.scene
+            .add_material_texture("ground_mat", "standard", "ground_texture")
+            .unwrap();
+        let wall_tex =
+            Texture::from_file(&device, &queue, "wall", &format!("{TEXTURES}/stonewall.jpg")).unwrap();
+        app.scene.add_texture("wall_texture", wall_tex).unwrap();
+        app.scene
+            .add_material_texture("wall_mat", "standard", "wall_texture")
+            .unwrap();
+
         // Large ground plane — will fade into fog at distance.
         app.scene
-            .create_mesh("ground_mesh", plane(80.0, 80.0, 1, 1), None)
+            .create_mesh("ground_mesh", plane(80.0, 80.0, 1, 1), Some("ground_mat"))
             .unwrap();
         app.scene.add_entity("ground", "ground_mesh").unwrap();
 
         // Row of cubes receding into the distance.
         app.scene
-            .create_mesh("cube_mesh", cube(1.0), None)
+            .create_mesh("cube_mesh", cube(1.0), Some("wall_mat"))
             .unwrap();
         for i in 0..15 {
             let z = -2.0 - i as f32 * 2.5;