fix: enable back-face culling — removes self-shadowing artifacts

Root cause: CULL_MODE_NONE in both main and shadow pipelines caused
back faces to render, creating self-shadowing acne on all objects:
- Shadow map: back faces written at closer depth → false shadows on front
- Main render: back faces visible through objects → wrong lighting

Fix:
- Main pipeline: CULL_MODE_BACK (was None) — only front faces render
- Shadow pipeline: CULL_MODE_BACK (was None) — only front faces cast shadows
- Removed normal flip hack (if dot(N,V) < 0 N = -N) — no longer needed
  with proper culling + correct CCW winding in cube.obj
- This is the standard approach: correct geometry + back-face culling

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

@@ -138,9 +138,6 @@ void main()
     float dist = length(toLight);
     vec3 L = toLight / max(dist, 0.001);
 
-    // Flip normal to face camera — fixes cubes with bad winding without breaking spheres
-    if (dot(N, V) < 0.0) N = -N;
-
     float attenuation = pow(clamp(1.0 - dist / max(lightRange, 0.001), 0.0, 1.0), 2.0);
     vec3 radiance = lightColor * lightIntensity * attenuation;
 

src/Engine.Graphics.Vulkan/VulkanPipeline.cs

@@ -102,7 +102,7 @@ internal sealed unsafe class VulkanPipeline : IDisposable
                 depthClampEnable = VkBool32.False,
                 rasterizerDiscardEnable = VkBool32.False,
                 polygonMode = VkPolygonMode.Fill,
-                cullMode = VkCullModeFlags.None,
+                cullMode = VkCullModeFlags.Back,
                 frontFace = VkFrontFace.CounterClockwise,
                 depthBiasEnable = VkBool32.False,
                 lineWidth = 1.0f,

src/Engine.Graphics.Vulkan/VulkanShadowMap.cs

@@ -275,7 +275,7 @@ internal sealed unsafe class VulkanShadowMap : IDisposable
                 depthClampEnable = VkBool32.False,
                 rasterizerDiscardEnable = VkBool32.False,
                 polygonMode = VkPolygonMode.Fill,
-                cullMode = VkCullModeFlags.None,
+                cullMode = VkCullModeFlags.Back,
                 frontFace = VkFrontFace.CounterClockwise,
                 depthBiasEnable = VkBool32.True,
                 lineWidth = 1.0f,