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);
 }

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

@@ -5,6 +5,7 @@ layout(location = 1) in vec3 inColor;
 layout(location = 2) in vec3 inNormal;
 
 layout(location = 0) out vec3 fragColor;
+layout(location = 1) out vec3 fragNormal;
 
 layout(row_major, set = 0, binding = 0) uniform CameraUBO {
     mat4 vp;
@@ -17,4 +18,5 @@ layout(row_major, push_constant) uniform PC {
 void main() {
     gl_Position = vp * pc.model * vec4(inPosition, 1.0);
     fragColor = inColor;
+    fragNormal = mat3(pc.model) * inNormal;
 }