fix: revert to CULL_MODE_NONE — no more holes in geometry

CULL_MODE_BACK was culling faces with inconsistent winding order in
sphere.obj, torus.obj, diamond.obj, cone.obj. Many faces disappeared,
making objects look like wireframe/grids.

Reverted both main and shadow pipelines to CULL_MODE_NONE.
Shadow acne is handled by low bias (0.0001) which works fine.

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.Back,
+                cullMode = VkCullModeFlags.None,
                 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.Back,
+                cullMode = VkCullModeFlags.None,
                 frontFace = VkFrontFace.CounterClockwise,
                 depthBiasEnable = VkBool32.True,
                 lineWidth = 1.0f,