fix: add normal-based diffuse lighting so cube is visible as 3D

- Fragment shader: directional light dot(normal, lightDir) with 0.2 ambient
- Vertex shader: pass normal through mat3(model) to fragment
- Same-color faces now have different shading based on orientation

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

@@ -1,8 +1,11 @@
 #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 lightDir = normalize(vec3(0.5, 0.8, 0.3));
+    float diff = max(dot(normalize(fragNormal), lightDir), 0.2);
+    outColor = vec4(fragColor * diff, 1.0);
 }