feat(renderer): add RT shadow resources and compute shader

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

crates/voltex_renderer/src/rt_shadow.rs

@@ -0,0 +1,89 @@
+use bytemuck::{Pod, Zeroable};
+
+/// Texture format used for the RT shadow output (single-channel float).
+pub const RT_SHADOW_FORMAT: wgpu::TextureFormat = wgpu::TextureFormat::R32Float;
+
+/// Uniform buffer for the RT shadow compute pass.
+///
+/// Layout (32 bytes, 16-byte aligned):
+///   light_direction [f32; 3] + _pad0: f32   → 16 bytes
+///   width: u32, height: u32, _pad1: [u32; 2] → 16 bytes
+#[repr(C)]
+#[derive(Copy, Clone, Debug, Pod, Zeroable)]
+pub struct RtShadowUniform {
+    pub light_direction: [f32; 3],
+    pub _pad0: f32,
+    pub width: u32,
+    pub height: u32,
+    pub _pad1: [u32; 2],
+}
+
+/// GPU resources for the RT shadow compute pass.
+pub struct RtShadowResources {
+    pub shadow_texture: wgpu::Texture,
+    pub shadow_view: wgpu::TextureView,
+    pub uniform_buffer: wgpu::Buffer,
+    pub width: u32,
+    pub height: u32,
+}
+
+impl RtShadowResources {
+    pub fn new(device: &wgpu::Device, width: u32, height: u32) -> Self {
+        let (shadow_texture, shadow_view) = create_shadow_texture(device, width, height);
+
+        let uniform_buffer = device.create_buffer(&wgpu::BufferDescriptor {
+            label: Some("RT Shadow Uniform Buffer"),
+            size: std::mem::size_of::<RtShadowUniform>() as u64,
+            usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
+            mapped_at_creation: false,
+        });
+
+        Self { shadow_texture, shadow_view, uniform_buffer, width, height }
+    }
+
+    /// Recreate the shadow texture when the window is resized.
+    pub fn resize(&mut self, device: &wgpu::Device, width: u32, height: u32) {
+        let (shadow_texture, shadow_view) = create_shadow_texture(device, width, height);
+        self.shadow_texture = shadow_texture;
+        self.shadow_view = shadow_view;
+        self.width = width;
+        self.height = height;
+    }
+}
+
+// ── Helpers ──────────────────────────────────────────────────────────────────
+
+fn create_shadow_texture(
+    device: &wgpu::Device,
+    width: u32,
+    height: u32,
+) -> (wgpu::Texture, wgpu::TextureView) {
+    let texture = device.create_texture(&wgpu::TextureDescriptor {
+        label: Some("RT Shadow Texture"),
+        size: wgpu::Extent3d {
+            width,
+            height,
+            depth_or_array_layers: 1,
+        },
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: wgpu::TextureDimension::D2,
+        format: RT_SHADOW_FORMAT,
+        usage: wgpu::TextureUsages::STORAGE_BINDING | wgpu::TextureUsages::TEXTURE_BINDING,
+        view_formats: &[],
+    });
+    let view = texture.create_view(&wgpu::TextureViewDescriptor::default());
+    (texture, view)
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_rt_shadow_uniform_size() {
+        assert_eq!(std::mem::size_of::<RtShadowUniform>(), 32);
+    }
+}