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/README.md

@@ -25,9 +25,42 @@ Examples gated behind a Cargo feature need the feature too:
 cargo run -p wsg-lib --example import --features import-obj
 ```
 
-All examples are **self-contained**: procedural textures, hard-coded geometries,
-no on-disk assets. All use the declarative API (`AppBuilder` + `AppHandler`)
-except `manual`, which demonstrates the low-level workflow instead.
+All examples are **self-contained**: hard-coded geometries, and textures that are
+either procedural or shipped in [`assets/textures/`](assets/textures/). All use the
+declarative API (`AppBuilder` + `AppHandler`) except `manual`, which demonstrates
+the low-level workflow instead.
+
+## Texture assets
+
+A few examples (the multi-mesh / multi-effect ones) use real image files from
+`assets/textures/`, loaded with `Texture::from_file`. The paths are resolved
+against `CARGO_MANIFEST_DIR` at compile time, so the examples work from **any
+working directory**:
+
+```rust
+const TEXTURES: &str = concat!(env!("CARGO_MANIFEST_DIR"), "/examples/assets/textures");
+Texture::from_file(&device, &queue, "label", &format!("{TEXTURES}/ground.jpeg"))
+```
+
+Assets used by the examples:
+
+| Asset | Size | Used by | Role |
+|-------|------|---------|------|
+| `uv_texture.jpg` | 437×438 | `cube`, `demo`, `shadow`, `dof`, `culling` | 8×8 UV atlas visualization (labelled cells + corner coordinates) — makes UV mapping and culling decisions explicit |
+| `ground.jpeg` | 512×512 | `demo`, `pbr`, `shadow`, `fog`, `dof` | Seamless ground albedo, tiled via the `Repeat` sampler |
+| `stonewall.jpg` | 300×225 | `fog` | Distinctive cube texture — the fog falloff reads clearly on it |
+| `cave.jpg` + `caveNormal.jpg` | 600×450 | `pbr` | Albedo + normal-map pair for the PBR normal-mapping demo |
+
+The remaining assets in the folder (`rock.jpg`, `seamlessRoad.jpg`, `stalag.jpg` /
+`stalagNormal.jpg`, `stars1.jpg`, sprite/heightmap PNGs, …) are available for
+experiments. Two notes:
+
+- `Texture` uploads to `Rgba8UnormSrgb` — correct for **albedo** maps (the GPU
+  sRGB-decodes on sample). A **normal map** is linear data, so `pbr` pre-encodes
+  its channels with the sRGB OETF before upload (`load_normal_map`): the GPU
+  decode then restores the original values (EOTF∘OETF = identity).
+- The texture sampler is `Linear` + `Repeat`, so any texture tiles automatically
+  when UVs exceed [0,1] (the 80×80 fog floor uses this to tile `ground.jpeg`).
 
 > **Where do the files live?** Examples live in subfolders
 > (`examples/<folder>/<name>.rs`). Cargo only auto-discovers top-level
@@ -56,6 +89,7 @@ except `manual`, which demonstrates the low-level workflow instead.
    name = "my_example"
    path = "examples/<folder>/my_example.rs"
    ```
-3. Keep it **self-contained**: procedural textures, hard-coded geometries, no
-   external assets.
+3. Keep it **self-contained**: hard-coded geometries; textures are procedural
+   or come from `assets/textures/` (resolved via `CARGO_MANIFEST_DIR`, see
+   *Texture assets* above).
 4. Document it in the folder's `README.md` (and in `docs/user/examples.md`).