feat: add shadow demo with directional light shadow mapping and 3x3 PCF

- Add Mat4::orthographic() to voltex_math for light projection
- Fix pbr_demo and multi_light_demo to provide shadow bind group (group 3)
  required by updated PBR pipeline (dummy shadow with size=0 disables it)
- Create shadow_demo with two-pass rendering: shadow depth pass using
  orthographic light projection, then PBR color pass with shadow sampling
- Scene: ground plane, 3 spheres, 2 cubes with directional light

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -13,6 +13,7 @@ members = [
     "examples/asset_demo",
     "examples/pbr_demo",
     "examples/multi_light_demo",
+    "examples/shadow_demo",
 ]
 
 [workspace.dependencies]