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>

examples/shadow_demo/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "shadow_demo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+voltex_math.workspace = true
+voltex_platform.workspace = true
+voltex_renderer.workspace = true
+wgpu.workspace = true
+winit.workspace = true
+bytemuck.workspace = true
+pollster.workspace = true
+env_logger.workspace = true
+log.workspace = true