fix(shadow): make Étape 14 shadow visible

The shadow pass was correct but the demo light was much too steep (52°
elevation), so the blocker's shadow fell in a ~0.5-unit sliver tight against
the cube's base and was invisible against the bright ground (offscreen pixel
probe found a single dark pixel). Verified with an offscreen probe using the
real Renderer::render_scene + shadow path:
  - steep front light   (0.6,1.1,0.6)  -> 1 dark pixel   (no visible shadow)
  - shallow side light  (1.0,0.3,0.0)  -> 17 107 pixels  (shadow pipeline OK)
  - tuned front-right   (1.0,0.5,0.0)  -> 16 979 pixels  (clear visible shadow)

The azimuth matters most: from the elevated front-right camera, a shadow cast
toward -z falls behind the cube and is occluded; one cast toward -x runs across
the ground to the left of the cube and reads clearly. Tuned light therefore sits
front-right and low (toward_light (1.0,0.5,0.0)), keeping the front faces lit
while casting a clearly visible PCF-softened shadow.

Also reapply the LessEqual comparison sampler fix (commit 39167ee had set it,
but was later reverted to GreaterEqual by 9a51ff7 while debugging; the probe
confirms LessEqual is the correct, non-inverted test). Correct 'rotating cube'
to 'cube' in README/ROADMAP (shadow_test scene is static).

lib/examples/shadow_test.rs

@@ -7,7 +7,9 @@
 //! directions are easy to read:
 //!
 //!   1. the **blocker** (cube) casts a directional shadow that stretches along
-//!      the ground opposite the light direction,
+//!      the ground opposite the light direction. The light sits at the camera's
+//!      front-right and low-ish, so its shadow runs clearly across the ground to
+//!      the left of the cube and is easy to see,
 //!   2. the shadow edge is **softened** by 3×3 PCF (no hard jagged border),
 //!   3. the lit faces are bright while the shadowed ground stays near-ambient,
 //!      proving the depth comparison is applied per-pixel.
@@ -118,9 +120,12 @@ impl wsg_lib::AppHandler for ShadowTest {
 
         // One directional light only: replace the default list.
         app.scene.clear_lights();
-        // Direction "from surface toward the light", i.e. the light source sits up and to
-        // the -x -z side, so the shadow is cast toward +x +z (toward the camera).
-        let toward_light = Vec3::new(-0.6, 1.1, -0.6).normalize();
+        // Direction "from surface toward the light": the light sits up and to the +x side
+        // (the camera's right), at a lowish elevation. Its shadow is then cast toward -x,
+        // running clearly across the ground to the left of the cube. A steeper or more
+        // frontal light would push the shadow tight against the cube's base or behind it,
+        // where it is occluded by the cube from this elevated front-right view.
+        let toward_light = Vec3::new(1.0, 0.5, 0.0).normalize();
         app.scene
             .add_directional_light(toward_light, [1.0, 0.98, 0.92], 1.6)
             .unwrap();