feat(renderer): add motion blur compute shader

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

crates/voltex_renderer/src/motion_blur.wgsl

@@ -0,0 +1,49 @@
+struct MotionBlurParams {
+    inv_view_proj: mat4x4<f32>,
+    prev_view_proj: mat4x4<f32>,
+    num_samples: u32,
+    strength: f32,
+    _pad: vec2<f32>,
+};
+
+@group(0) @binding(0) var color_tex: texture_2d<f32>;
+@group(0) @binding(1) var depth_tex: texture_depth_2d;
+@group(0) @binding(2) var output_tex: texture_storage_2d<rgba16float, write>;
+@group(0) @binding(3) var<uniform> params: MotionBlurParams;
+
+@compute @workgroup_size(16, 16)
+fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
+    let dims = textureDimensions(color_tex);
+    if (gid.x >= dims.x || gid.y >= dims.y) { return; }
+
+    let pos = vec2<i32>(gid.xy);
+    let uv = (vec2<f32>(gid.xy) + 0.5) / vec2<f32>(dims);
+
+    // Reconstruct world position from depth
+    let depth = textureLoad(depth_tex, pos, 0);
+    let ndc = vec4<f32>(uv.x * 2.0 - 1.0, 1.0 - uv.y * 2.0, depth, 1.0);
+    let world = params.inv_view_proj * ndc;
+    let world_pos = world.xyz / world.w;
+
+    // Project to previous frame
+    let prev_clip = params.prev_view_proj * vec4<f32>(world_pos, 1.0);
+    let prev_ndc = prev_clip.xyz / prev_clip.w;
+    let prev_uv = vec2<f32>(prev_ndc.x * 0.5 + 0.5, 1.0 - (prev_ndc.y * 0.5 + 0.5));
+
+    // Velocity = current_uv - prev_uv
+    let velocity = (uv - prev_uv) * params.strength;
+
+    // Sample along velocity direction
+    var color = vec4<f32>(0.0);
+    let n = params.num_samples;
+    for (var i = 0u; i < n; i++) {
+        let t = f32(i) / f32(n - 1u) - 0.5; // -0.5 to 0.5
+        let sample_uv = uv + velocity * t;
+        let sample_pos = vec2<i32>(sample_uv * vec2<f32>(dims));
+        let clamped = clamp(sample_pos, vec2<i32>(0), vec2<i32>(dims) - 1);
+        color += textureLoad(color_tex, clamped, 0);
+    }
+    color /= f32(n);
+
+    textureStore(output_tex, pos, color);
+}