feat: PBR shading — Cook-Torrance BRDF with ACES tonemapping

- Vertex shader: passes world position, world normal, albedo to fragment
- Fragment shader: full PBR implementation
  - D term: Trowbridge-Reitz GGX distribution
  - G term: Smith geometry with Schlick-GGX
  - F term: Schlick Fresnel approximation
  - kD/kS split based on metallic
  - Directional light (0.5, 0.8, 0.3) with warm color
  - Ambient term (0.15, 0.18, 0.22) for fill light
  - ACES filmic tonemapping
  - Gamma 2.2 correction
- Fixed roughness=0.5, metallic=0.1 (per-object materials = future)
- No C# code changes — pure shader upgrade

src/Engine.Graphics.Vulkan/Shaders/triangle.frag

@@ -1,14 +1,93 @@
 #version 450
 
-layout(location = 0) in vec3 fragColor;
+layout(location = 0) in vec3 fragWorldPos;
 layout(location = 1) in vec3 fragNormal;
+layout(location = 2) in vec3 fragAlbedo;
+
 layout(location = 0) out vec4 outColor;
 
-void main() {
+const vec3 LIGHT_DIR = normalize(vec3(0.5, 0.8, 0.3));
+const vec3 LIGHT_COLOR = vec3(1.0, 0.95, 0.85);
+const vec3 AMBIENT = vec3(0.15, 0.18, 0.22);
+
+const float PI = 3.14159265359;
+
+float distributionGGX(vec3 N, vec3 H, float roughness)
+{
+    float a = roughness * roughness;
+    float a2 = a * a;
+    float NdotH = max(dot(N, H), 0.0);
+    float NdotH2 = NdotH * NdotH;
+    float num = a2;
+    float denom = NdotH2 * (a2 - 1.0) + 1.0;
+    denom = PI * denom * denom;
+    return num / denom;
+}
+
+float geometrySchlickGGX(float NdotV, float roughness)
+{
+    float r = roughness + 1.0;
+    float k = (r * r) / 8.0;
+    return NdotV / (NdotV * (1.0 - k) + k);
+}
+
+float geometrySmith(vec3 N, vec3 V, vec3 L, float roughness)
+{
+    float NdotV = max(dot(N, V), 0.0);
+    float NdotL = max(dot(N, L), 0.0);
+    float ggx2 = geometrySchlickGGX(NdotV, roughness);
+    float ggx1 = geometrySchlickGGX(NdotL, roughness);
+    return ggx2 * ggx1;
+}
+
+vec3 fresnelSchlick(float cosTheta, vec3 F0)
+{
+    return F0 + (1.0 - F0) * pow(clamp(1.0 - cosTheta, 0.0, 1.0), 5.0);
+}
+
+vec3 acesTonemap(vec3 color)
+{
+    float a = 2.51;
+    float b = 0.03;
+    float c = 2.43;
+    float d = 0.59;
+    float e = 0.14;
+    return clamp((color * (a * color + b)) / (color * (c * color + d) + e), 0.0, 1.0);
+}
+
+void main()
+{
     vec3 N = normalize(fragNormal);
-    vec3 L = normalize(vec3(0.5, 0.8, 0.3));
-    float diff = max(dot(N, L), 0.0);
-    float ambient = 0.25;
-    vec3 color = fragColor * (ambient + diff * 0.75);
+    vec3 V = normalize(-fragWorldPos);
+
+    vec3 albedo = fragAlbedo;
+    float roughness = 0.5;
+    float metallic = 0.1;
+
+    vec3 F0 = mix(vec3(0.04), albedo, metallic);
+
+    vec3 L = LIGHT_DIR;
+    vec3 H = normalize(V + L);
+
+    float NDF = distributionGGX(N, H, roughness);
+    float G = geometrySmith(N, V, L, roughness);
+    vec3 F = fresnelSchlick(max(dot(H, V), 0.0), F0);
+
+    vec3 numerator = NDF * G * F;
+    float denominator = 4.0 * max(dot(N, V), 0.0) * max(dot(N, L), 0.0) + 0.0001;
+    vec3 specular = numerator / denominator;
+
+    vec3 kS = F;
+    vec3 kD = (vec3(1.0) - kS) * (1.0 - metallic);
+
+    float NdotL = max(dot(N, L), 0.0);
+    vec3 Lo = (kD * albedo / PI + specular) * LIGHT_COLOR * NdotL;
+
+    vec3 ambient = AMBIENT * albedo;
+    vec3 color = ambient + Lo;
+
+    color = acesTonemap(color);
+    color = pow(color, vec3(1.0 / 2.2));
+
     outColor = vec4(color, 1.0);
 }

src/Engine.Graphics.Vulkan/Shaders/triangle.vert

@@ -4,8 +4,9 @@ layout(location = 0) in vec3 inPosition;
 layout(location = 1) in vec3 inColor;
 layout(location = 2) in vec3 inNormal;
 
-layout(location = 0) out vec3 fragColor;
+layout(location = 0) out vec3 fragWorldPos;
 layout(location = 1) out vec3 fragNormal;
+layout(location = 2) out vec3 fragAlbedo;
 
 layout(set = 0, binding = 0) uniform CameraUBO {
     mat4 vp;
@@ -16,7 +17,9 @@ layout(push_constant) uniform PC {
 } pc;
 
 void main() {
-    gl_Position = vp * pc.model * vec4(inPosition, 1.0);
-    fragColor = inColor;
+    vec4 worldPos = pc.model * vec4(inPosition, 1.0);
+    gl_Position = vp * worldPos;
+    fragWorldPos = worldPos.xyz;
     fragNormal = mat3(pc.model) * inNormal;
+    fragAlbedo = inColor;
 }