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/meshes/cube.rs

@@ -1,4 +1,5 @@
-//! A lit unit cube that rotates, **textured** with a procedural checkerboard via the diffuse path
+//! A lit unit cube that rotates, **textured** with the `uv_texture.jpg` asset (an 8×8 UV atlas
+//! visualization — each labelled cell shows exactly where a face's UVs land) via the diffuse path
 //! (bind group `@group(2)`).
 //!
 //! A 3D mesh with Phong lighting on screen — the library's 3D showcase.
@@ -7,7 +8,9 @@
 //! `add_material_shader`/`add_material_texture` + `create_mesh` + `add_entity`. The mesh
 //! is declared from a **`Geometry`** (positions, normals, indices). A texture is
 //! registered by id (`add_texture`) and a textured material bound to it (`add_material_texture`);
-//! the texture is generated *procedurally* (RGBA 8×8 checkerboard) to stay self-contained, no on-disk asset.
+//! here the texture is a **file asset** (`assets/textures/uv_texture.jpg`, loaded via
+//! `Texture::from_file`) — the path is resolved against `CARGO_MANIFEST_DIR` so the example
+//! works from any working directory.
 //! The default active camera (`Scene::default`, position (0,0,3), fov 45°) frames the cube, and
 //! `AppHandler::update` rotates the entity via `set_entity_transform` each frame.
 use glam::{Quat, Vec3};
@@ -17,27 +20,15 @@ use wsg_lib::mesh::cube;
 use wsg_lib::resources::Texture;
 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");
+
 /// Demo handler: rotates the textured cube in `update`.
 struct Cube {
     /// Cumulative rotation angle (radians), incremented each frame.
     angle: f32,
 }
 
-/// Generates a *procedural* RGBA 8×8 checkerboard (white/brick), no on-disk asset, to texture the
-/// cube. Returned as a raw RGBA8 `Vec<u8>`, loadable via `Texture::from_rgba8`.
-fn checkerboard_rgba() -> Vec<u8> {
-    const SIZE: u32 = 8;
-    let mut rgba = Vec::with_capacity((SIZE * SIZE * 4) as usize);
-    for y in 0..SIZE {
-        for x in 0..SIZE {
-            let even = (x + y) % 2 == 0;
-            let (r, g, b) = if even { (255, 255, 255) } else { (190, 40, 40) };
-            rgba.extend_from_slice(&[r, g, b, 255]);
-        }
-    }
-    rgba
-}
-
 impl AppHandler for Cube {
     fn setup(&mut self, app: &mut wsg_lib::App) {
         // Phong shader `standard` (carries the frame + object + texture bind groups).
@@ -45,17 +36,22 @@ impl AppHandler for Cube {
             .register_shader("standard", wsg_lib::utils::STANDARD_SHADER_PATH)
             .unwrap();
 
-        // Builds the checkerboard texture with the Context's device/queue (via `app.context()`), then
+        // Loads the `uv_texture.jpg` asset with the Context's device/queue (via `app.context()`), then
         // registers it in the scene by id; a textured material is then bound to that id.
         let (device, queue) = {
             let ctx = app.context();
             (ctx.device.clone(), ctx.queue.clone())
         };
-        let texture =
-            Texture::from_rgba8(&device, &queue, 8, 8, &checkerboard_rgba(), "checker").unwrap();
-        app.scene.add_texture("checker_texture", texture).unwrap();
+        let texture = Texture::from_file(
+            &device,
+            &queue,
+            "uv_atlas",
+            &format!("{TEXTURES}/uv_texture.jpg"),
+        )
+        .unwrap();
+        app.scene.add_texture("uv_texture", texture).unwrap();
         app.scene
-            .add_material_texture("cube_material", "standard", "checker_texture")
+            .add_material_texture("cube_material", "standard", "uv_texture")
             .unwrap();
 
         app.scene