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_shader.wgsl

@@ -0,0 +1,79 @@
+// RT Shadow compute shader.
+// Reads world-space position and normal from the G-Buffer, then fires a
+// shadow ray against the TLAS to determine per-pixel visibility.
+// Output is 1.0 (lit) or 0.0 (shadowed) stored in an R32Float texture.
+
+// ── Group 0: G-Buffer inputs ──────────────────────────────────────────────────
+
+@group(0) @binding(0) var t_position: texture_2d<f32>;
+@group(0) @binding(1) var t_normal:   texture_2d<f32>;
+
+// ── Group 1: RT data ─────────────────────────────────────────────────────────
+
+@group(1) @binding(0) var tlas:          acceleration_structure;
+@group(1) @binding(1) var t_shadow_out:  texture_storage_2d<r32float, write>;
+
+struct RtShadowUniform {
+    light_direction: vec3<f32>,
+    _pad0: f32,
+    width: u32,
+    height: u32,
+    _pad1: vec2<u32>,
+};
+
+@group(1) @binding(2) var<uniform> uniforms: RtShadowUniform;
+
+// ── Compute entry point ───────────────────────────────────────────────────────
+
+@compute @workgroup_size(8, 8)
+fn cs_main(@builtin(global_invocation_id) gid: vec3<u32>) {
+    let coord = vec2<i32>(i32(gid.x), i32(gid.y));
+
+    // Bounds check
+    if gid.x >= uniforms.width || gid.y >= uniforms.height {
+        return;
+    }
+
+    // Read world position from G-Buffer
+    let world_pos = textureLoad(t_position, coord, 0).xyz;
+
+    // Background pixel: skip (position is (0,0,0) for skybox pixels)
+    if dot(world_pos, world_pos) < 0.001 {
+        textureStore(t_shadow_out, coord, vec4<f32>(1.0, 0.0, 0.0, 0.0));
+        return;
+    }
+
+    // Read and decode normal — G-Buffer stores N * 0.5 + 0.5
+    let normal_encoded = textureLoad(t_normal, coord, 0).rgb;
+    let N = normalize(normal_encoded * 2.0 - 1.0);
+
+    // Ray: from surface towards the light, biased along normal to avoid self-intersection
+    let ray_origin = world_pos + N * 0.01;
+    let ray_dir    = normalize(-uniforms.light_direction);
+
+    // Ray query: check for any occluder between surface and "infinity"
+    var rq: ray_query;
+    rayQueryInitialize(
+        &rq,
+        tlas,
+        RayDesc(
+            RAY_FLAG_TERMINATE_ON_FIRST_HIT | RAY_FLAG_SKIP_CLOSEST_HIT_SHADER,
+            0xFF,
+            0.0001,
+            1.0e6,
+            ray_origin,
+            ray_dir,
+        ),
+    );
+
+    // Advance until the query is done (with TERMINATE_ON_FIRST_HIT this is at most one step)
+    while rayQueryProceed(&rq) {}
+
+    // If anything was hit, the pixel is in shadow
+    var shadow: f32 = 1.0;
+    if rayQueryGetCommittedIntersectionType(&rq) != RAY_QUERY_INTERSECTION_NONE {
+        shadow = 0.0;
+    }
+
+    textureStore(t_shadow_out, coord, vec4<f32>(shadow, 0.0, 0.0, 0.0));
+}