feat: diffuse lighting — normals passed to fragment shader

- Vertex shader: pass normal through mat3(model) to fragment
- Fragment shader: directional light (0.5, 0.8, 0.3) with 0.25 ambient
  + 0.75 diffuse = faces now have depth and 3D form
- Torus knot has smooth normals (vn in OBJ), cubes have face normals
- 0 validation errors, ~2300 FPS

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

@@ -1,8 +1,14 @@
 #version 450
 
 layout(location = 0) in vec3 fragColor;
+layout(location = 1) in vec3 fragNormal;
 layout(location = 0) out vec4 outColor;
 
 void main() {
-    outColor = vec4(fragColor, 1.0);
+    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);
+    outColor = vec4(color, 1.0);
 }