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.lock

@@ -989,6 +989,21 @@ dependencies = [
  "winit",
 ]
 
+[[package]]
+name = "multi_light_demo"
+version = "0.1.0"
+dependencies = [
+ "bytemuck",
+ "env_logger",
+ "log",
+ "pollster",
+ "voltex_math",
+ "voltex_platform",
+ "voltex_renderer",
+ "wgpu",
+ "winit",
+]
+
 [[package]]
 name = "naga"
 version = "28.0.0"
@@ -1697,6 +1712,21 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "shadow_demo"
+version = "0.1.0"
+dependencies = [
+ "bytemuck",
+ "env_logger",
+ "log",
+ "pollster",
+ "voltex_math",
+ "voltex_platform",
+ "voltex_renderer",
+ "wgpu",
+ "winit",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"