fix(shadow): read directional light direction from position_dir, not dir_angle

For directional lights dir_angle is Vec4::ZERO, so the old code built the
shadow light_view_proj from dir=(0,0,0), making eye==target and look_at_mat4
degenerate -> NaN light_view_proj -> compute_shadow -> fully black frame.

Directional direction lives in position_dir.xyz (surface->light); the shadow
camera looks along the light's travel direction (light->scene), i.e. the
negation, matching the DRAFT spec.

lib/src/core/renderer.rs

@@ -336,9 +336,18 @@ impl Renderer {
         let light = lights.get(index)?;
         // Directional and spot lights carry a direction; point lights would need a 6-face cubemap
         // shadow, which is out of scope (D6), so we reject them.
+        // Directional lights carry their direction in `position_dir.xyz` (from the surface toward
+        // the light, see `directional_light`/shader); `dir_angle` is zero for them. Spot lights
+        // carry the cone axis (from the light toward the scene) in `dir_angle.xyz`. The shadow
+        // camera must look along the light's **travel direction** (light → scene), i.e. the negation
+        // of the surface→light vector for directional lights.
         let dir = match light.light_type() {
-            crate::resources::LightType::Directional
-            | crate::resources::LightType::Spot { .. } => Vec3::new(
+            crate::resources::LightType::Directional => Vec3::new(
+                -light.position_dir.x,
+                -light.position_dir.y,
+                -light.position_dir.z,
+            ),
+            crate::resources::LightType::Spot { .. } => Vec3::new(
                 light.dir_angle.x,
                 light.dir_angle.y,
                 light.dir_angle.z,