feat(renderer): add ShadowMap, shadow depth shader, and shadow pipeline

Implements ShadowMap (2048x2048 Depth32Float texture with comparison sampler),
shadow_shader.wgsl (depth-only vertex shader), shadow_pipeline (front-face
culling, depth bias constant=2/slope=2.0), and associated uniform types.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/voltex_renderer/src/shadow.rs

@@ -0,0 +1,125 @@
+use bytemuck::{Pod, Zeroable};
+
+pub const SHADOW_MAP_SIZE: u32 = 2048;
+pub const SHADOW_FORMAT: wgpu::TextureFormat = wgpu::TextureFormat::Depth32Float;
+
+pub struct ShadowMap {
+    pub texture: wgpu::Texture,
+    pub view: wgpu::TextureView,
+    pub sampler: wgpu::Sampler,
+}
+
+impl ShadowMap {
+    pub fn new(device: &wgpu::Device) -> Self {
+        let texture = device.create_texture(&wgpu::TextureDescriptor {
+            label: Some("Shadow Map Texture"),
+            size: wgpu::Extent3d {
+                width: SHADOW_MAP_SIZE,
+                height: SHADOW_MAP_SIZE,
+                depth_or_array_layers: 1,
+            },
+            mip_level_count: 1,
+            sample_count: 1,
+            dimension: wgpu::TextureDimension::D2,
+            format: SHADOW_FORMAT,
+            usage: wgpu::TextureUsages::RENDER_ATTACHMENT | wgpu::TextureUsages::TEXTURE_BINDING,
+            view_formats: &[],
+        });
+
+        let view = texture.create_view(&wgpu::TextureViewDescriptor::default());
+
+        let sampler = device.create_sampler(&wgpu::SamplerDescriptor {
+            label: Some("Shadow Map Sampler"),
+            address_mode_u: wgpu::AddressMode::ClampToEdge,
+            address_mode_v: wgpu::AddressMode::ClampToEdge,
+            address_mode_w: wgpu::AddressMode::ClampToEdge,
+            mag_filter: wgpu::FilterMode::Linear,
+            min_filter: wgpu::FilterMode::Linear,
+            mipmap_filter: wgpu::MipmapFilterMode::Nearest,
+            compare: Some(wgpu::CompareFunction::LessEqual),
+            ..Default::default()
+        });
+
+        Self { texture, view, sampler }
+    }
+
+    pub fn bind_group_layout(device: &wgpu::Device) -> wgpu::BindGroupLayout {
+        device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
+            label: Some("Shadow Bind Group Layout"),
+            entries: &[
+                // binding 0: depth texture
+                wgpu::BindGroupLayoutEntry {
+                    binding: 0,
+                    visibility: wgpu::ShaderStages::FRAGMENT,
+                    ty: wgpu::BindingType::Texture {
+                        sample_type: wgpu::TextureSampleType::Depth,
+                        view_dimension: wgpu::TextureViewDimension::D2,
+                        multisampled: false,
+                    },
+                    count: None,
+                },
+                // binding 1: comparison sampler
+                wgpu::BindGroupLayoutEntry {
+                    binding: 1,
+                    visibility: wgpu::ShaderStages::FRAGMENT,
+                    ty: wgpu::BindingType::Sampler(wgpu::SamplerBindingType::Comparison),
+                    count: None,
+                },
+                // binding 2: ShadowUniform buffer
+                wgpu::BindGroupLayoutEntry {
+                    binding: 2,
+                    visibility: wgpu::ShaderStages::FRAGMENT,
+                    ty: wgpu::BindingType::Buffer {
+                        ty: wgpu::BufferBindingType::Uniform,
+                        has_dynamic_offset: false,
+                        min_binding_size: wgpu::BufferSize::new(
+                            std::mem::size_of::<ShadowUniform>() as u64,
+                        ),
+                    },
+                    count: None,
+                },
+            ],
+        })
+    }
+
+    pub fn create_bind_group(
+        &self,
+        device: &wgpu::Device,
+        layout: &wgpu::BindGroupLayout,
+        shadow_uniform_buffer: &wgpu::Buffer,
+    ) -> wgpu::BindGroup {
+        device.create_bind_group(&wgpu::BindGroupDescriptor {
+            label: Some("Shadow Bind Group"),
+            layout,
+            entries: &[
+                wgpu::BindGroupEntry {
+                    binding: 0,
+                    resource: wgpu::BindingResource::TextureView(&self.view),
+                },
+                wgpu::BindGroupEntry {
+                    binding: 1,
+                    resource: wgpu::BindingResource::Sampler(&self.sampler),
+                },
+                wgpu::BindGroupEntry {
+                    binding: 2,
+                    resource: shadow_uniform_buffer.as_entire_binding(),
+                },
+            ],
+        })
+    }
+}
+
+#[repr(C)]
+#[derive(Copy, Clone, Debug, Pod, Zeroable)]
+pub struct ShadowUniform {
+    pub light_view_proj: [[f32; 4]; 4],
+    pub shadow_map_size: f32,
+    pub shadow_bias: f32,
+    pub _padding: [f32; 2],
+}
+
+#[repr(C)]
+#[derive(Copy, Clone, Debug, Pod, Zeroable)]
+pub struct ShadowPassUniform {
+    pub light_vp_model: [[f32; 4]; 4],
+}