Implement shadow mapping for directional lights:
- Scene::set_shadow_caster(Option<usize>) selects the shadow-casting light
by packed frame-array index (None disables; point lights rejected at render).
- Lights::get(index) resolves a packed index across the directional/point/spot lists.
- Renderer allocates a shadow depth map, comparison sampler, group-3 bind groups,
shadow uniform buffer and shadow pipeline; render_scene does a depth-only
shadow pass before the main pass; compute_shadow_light_view_proj builds an
orthographic light-space frustum from the scene radius.
- standard_shader: shadow_light_index/light_view_proj/shadow_params uniforms,
@group(3) depth map + comparison sampler, 3x3 PCF compute_shadow().
- shadow_shader: path/vertex shader with attribute layout matching the shared
vertex buffer (only position consumed).
- shadow_test example: directional shadow caster casts a PCF-softened shadow
onto a ground slab; documented in examples README.
Jérôme Bousquié